diff --git a/docs/whats-new.md b/docs/whats-new.md
index 085562a..1dec021 100644
--- a/docs/whats-new.md
+++ b/docs/whats-new.md
@@ -8,6 +8,7 @@
 - update dependencies and code to hatyan 2.8.0 in [#28](https://github.com/Deltares-research/kenmerkendewaarden/pull/28)
 - added neaptide tidal indicators for extremes in [#34](https://github.com/Deltares-research/kenmerkendewaarden/pull/34)
 - used threshold frequency instead of fixed index in `kw.overschrijding.blend_distributions` in [#38](https://github.com/Deltares-research/kenmerkendewaarden/pull/38)
+- dropped timezones consistently in `kw.calc_wltidalindicators()` and `kw.calc_HWLWtidalindicators()` to increase performance [#41](https://github.com/Deltares-research/kenmerkendewaarden/pull/41)
 
 
 ## 0.1.0 (2024-03-11)
diff --git a/kenmerkendewaarden/tidalindicators.py b/kenmerkendewaarden/tidalindicators.py
index 6b038ea..7155a01 100644
--- a/kenmerkendewaarden/tidalindicators.py
+++ b/kenmerkendewaarden/tidalindicators.py
@@ -32,6 +32,10 @@ def calc_HWLWtidalindicators(data_pd_HWLW_all, min_count=None):
         DESCRIPTION.
 
     """
+    # dropping the timezone makes the code below much faster and gives equal results: https://github.com/pandas-dev/pandas/issues/58956
+    if data_pd_HWLW_all.index.tz is not None:
+        data_pd_HWLW_all = data_pd_HWLW_all.tz_localize(None)
+        
     if len(data_pd_HWLW_all['HWLWcode'].unique()) > 2: #aggers are present
         # TODO: this drops first value if it is a 3/4/5 LW, should be fixed: https://github.com/Deltares/hatyan/issues/311
         data_pd_HWLW_12 = calc_HWLW12345to12(data_pd_HWLW_all) #convert 12345 to 12 by taking minimum of 345 as 2 (laagste laagwater)
@@ -43,18 +47,18 @@ def calc_HWLWtidalindicators(data_pd_HWLW_all, min_count=None):
     data_pd_LW = data_pd_HWLW_12.loc[data_pd_HWLW_12['HWLWcode']==2]
     
     #count HWLW values per year/month
-    HWLW_count_peryear = data_pd_HWLW_12.groupby(pd.PeriodIndex(data_pd_HWLW_12.index, freq="y"))['values'].count()
-    HWLW_count_permonth = data_pd_HWLW_12.groupby(pd.PeriodIndex(data_pd_HWLW_12.index, freq="m"))['values'].count()
+    HWLW_count_peryear = data_pd_HWLW_12.groupby(pd.PeriodIndex(data_pd_HWLW_12.index, freq="Y"))['values'].count()
+    HWLW_count_permonth = data_pd_HWLW_12.groupby(pd.PeriodIndex(data_pd_HWLW_12.index, freq="M"))['values'].count()
     
     #yearmean HWLW from HWLW values #maybe also add *_mean_permonth
-    HW_mean_peryear = data_pd_HW.groupby(pd.PeriodIndex(data_pd_HW.index, freq="y"))[['values']].mean()
-    LW_mean_peryear = data_pd_LW.groupby(pd.PeriodIndex(data_pd_LW.index, freq="y"))[['values']].mean()
+    HW_mean_peryear = data_pd_HW.groupby(pd.PeriodIndex(data_pd_HW.index, freq="Y"))[['values']].mean()
+    LW_mean_peryear = data_pd_LW.groupby(pd.PeriodIndex(data_pd_LW.index, freq="Y"))[['values']].mean()
     
     #derive GHHW/GHWS (gemiddeld hoogwater springtij) per month
-    HW_monthmax_permonth = data_pd_HW.groupby(pd.PeriodIndex(data_pd_HW.index, freq="m"))[['values']].max() #proxy for HW at spring tide
-    LW_monthmin_permonth = data_pd_LW.groupby(pd.PeriodIndex(data_pd_LW.index, freq="m"))[['values']].min() #proxy for LW at spring tide
-    HW_monthmin_permonth = data_pd_HW.groupby(pd.PeriodIndex(data_pd_HW.index, freq="m"))[['values']].min() #proxy for HW at neap tide
-    LW_monthmax_permonth = data_pd_LW.groupby(pd.PeriodIndex(data_pd_LW.index, freq="m"))[['values']].max() #proxy for LW at neap tide
+    HW_monthmax_permonth = data_pd_HW.groupby(pd.PeriodIndex(data_pd_HW.index, freq="M"))[['values']].max() #proxy for HW at spring tide
+    LW_monthmin_permonth = data_pd_LW.groupby(pd.PeriodIndex(data_pd_LW.index, freq="M"))[['values']].min() #proxy for LW at spring tide
+    HW_monthmin_permonth = data_pd_HW.groupby(pd.PeriodIndex(data_pd_HW.index, freq="M"))[['values']].min() #proxy for HW at neap tide
+    LW_monthmax_permonth = data_pd_LW.groupby(pd.PeriodIndex(data_pd_LW.index, freq="M"))[['values']].max() #proxy for LW at neap tide
     
     #replace invalids with nan (in case of too less values per month or year)
     if min_count is not None:
@@ -67,10 +71,10 @@ def calc_HWLWtidalindicators(data_pd_HWLW_all, min_count=None):
         LW_monthmax_permonth.loc[HWLW_count_permonth<min_count_permonth] = np.nan
     
     #derive GHHW/GHWS (gemiddeld hoogwater springtij)
-    HW_monthmax_mean_peryear = HW_monthmax_permonth.groupby(pd.PeriodIndex(HW_monthmax_permonth.index, freq="y"))[['values']].mean()
-    LW_monthmin_mean_peryear = LW_monthmin_permonth.groupby(pd.PeriodIndex(LW_monthmin_permonth.index, freq="y"))[['values']].mean()
-    HW_monthmin_mean_peryear = HW_monthmin_permonth.groupby(pd.PeriodIndex(HW_monthmin_permonth.index, freq="y"))[['values']].mean()
-    LW_monthmax_mean_peryear = LW_monthmax_permonth.groupby(pd.PeriodIndex(LW_monthmax_permonth.index, freq="y"))[['values']].mean()
+    HW_monthmax_mean_peryear = HW_monthmax_permonth.groupby(pd.PeriodIndex(HW_monthmax_permonth.index, freq="Y"))[['values']].mean()
+    LW_monthmin_mean_peryear = LW_monthmin_permonth.groupby(pd.PeriodIndex(LW_monthmin_permonth.index, freq="Y"))[['values']].mean()
+    HW_monthmin_mean_peryear = HW_monthmin_permonth.groupby(pd.PeriodIndex(HW_monthmin_permonth.index, freq="Y"))[['values']].mean()
+    LW_monthmax_mean_peryear = LW_monthmax_permonth.groupby(pd.PeriodIndex(LW_monthmax_permonth.index, freq="Y"))[['values']].mean()
     
     dict_HWLWtidalindicators = {'HW_mean':data_pd_HW['values'].mean(), #GHW
                                 'LW_mean':data_pd_LW['values'].mean(), #GLW
@@ -109,16 +113,17 @@ def calc_wltidalindicators(data_wl_pd, min_count=None):
         DESCRIPTION.
 
     """
-    if hasattr(data_wl_pd.index[0],'tz'): #timezone present in index
-        data_wl_pd.index = data_wl_pd.index.tz_localize(None)
+    # dropping the timezone makes the code below much faster and gives equal results: https://github.com/pandas-dev/pandas/issues/58956
+    if data_wl_pd.index.tz is not None:
+        data_wl_pd = data_wl_pd.tz_localize(None)
     
     #count wl values per year/month
-    wl_count_peryear = data_wl_pd.groupby(pd.PeriodIndex(data_wl_pd.index, freq="y"))['values'].count()
-    wl_count_permonth = data_wl_pd.groupby(pd.PeriodIndex(data_wl_pd.index, freq="m"))['values'].count()
+    wl_count_peryear = data_wl_pd.groupby(pd.PeriodIndex(data_wl_pd.index, freq="Y"))['values'].count()
+    wl_count_permonth = data_wl_pd.groupby(pd.PeriodIndex(data_wl_pd.index, freq="M"))['values'].count()
     
     #yearmean wl from wl values
-    wl_mean_peryear = data_wl_pd.groupby(pd.PeriodIndex(data_wl_pd.index, freq="y"))[['values']].mean()
-    wl_mean_permonth = data_wl_pd.groupby(pd.PeriodIndex(data_wl_pd.index, freq="m"))[['values']].mean()
+    wl_mean_peryear = data_wl_pd.groupby(pd.PeriodIndex(data_wl_pd.index, freq="Y"))[['values']].mean()
+    wl_mean_permonth = data_wl_pd.groupby(pd.PeriodIndex(data_wl_pd.index, freq="M"))[['values']].mean()
     
     #replace invalids with nan (in case of too less values per month or year)
     if min_count is not None:
diff --git a/tests/test_tidalindicators.py b/tests/test_tidalindicators.py
index 18e9c2c..36b1efc 100644
--- a/tests/test_tidalindicators.py
+++ b/tests/test_tidalindicators.py
@@ -5,6 +5,7 @@
 import hatyan
 import kenmerkendewaarden as kw
 import numpy as np
+import pandas as pd
 
 dir_tests = os.path.dirname(__file__) #F9 doesnt work, only F5 (F5 also only method to reload external definition scripts)
 dir_testdata = os.path.join(dir_tests,'testdata')
@@ -20,3 +21,107 @@ def test_calc_HWLWtidalrange():
     vals_expected = np.array([4.  , 4.  , 4.1 , 4.1 , 3.77, 3.77, 3.89, 3.89, 3.5 , 3.5 ])
     assert len(ranges) == 1411
     assert np.allclose(ranges[-10:], vals_expected)
+
+
+@pytest.fixture(scope="session")
+def prediction():
+    comp = pd.DataFrame({"A": [1, 0.5, 0.2],
+                         "phi_deg": [10,15,20]}, 
+                        index=["M2","M4","S2"])
+    comp.attrs["nodalfactors"] = True
+    comp.attrs["fu_alltimes"] = True
+    comp.attrs["xfac"] = False
+    comp.attrs["source"] = "schureman"
+    comp.attrs["tzone"] = None # TODO: required but this is a bug: https://github.com/Deltares/hatyan/issues/317
+    dtindex = pd.date_range("2020-01-01","2024-01-01", freq="10min")
+    prediction = hatyan.prediction(comp, times=dtindex)
+    return prediction
+
+
+@pytest.fixture(scope="session")
+def prediction_extremes(prediction):
+    prediction_extremes = hatyan.calc_HWLW(prediction)
+    return prediction_extremes
+
+
+@pytest.mark.unittest
+def test_calc_HWLWtidalindicators(prediction):
+    wl_stats = kw.calc_wltidalindicators(prediction)
+    wl_stats_tzone = kw.calc_wltidalindicators(prediction.tz_localize("UTC+01:00"))
+    
+    expected_keys = ['wl_mean_peryear', 'wl_mean_permonth']
+    for key in expected_keys:
+        assert key in wl_stats.keys()
+        assert (wl_stats[key] == wl_stats_tzone[key]).all()
+    
+    wl_mean_peryear_expected = np.array([-1.74201105e-04,  4.19964454e-04, -6.46875301e-05, -2.36333457e-04,
+           -5.63995348e-01])
+    wl_mean_permonth_expected = np.array([ 9.62018771e-04, -3.31675667e-04,  8.53396688e-04,  3.16123043e-04,
+            1.13262094e-03,  3.56346658e-04,  4.53580440e-04, -1.14071322e-03,
+           -7.03490275e-04, -2.39556946e-03, -6.59549426e-04, -9.43460292e-04,
+            6.51602796e-04, -7.09547154e-04,  5.96494198e-04,  3.99839817e-04,
+            1.22594977e-03,  3.34308253e-04,  8.58938332e-04,  1.00068126e-03,
+            3.79093532e-04,  9.16718937e-04,  8.28879097e-05, -8.22305918e-04,
+           -2.17133133e-03,  2.47645095e-03, -2.13131823e-03, -7.85535462e-04,
+           -1.76888980e-03, -2.41317388e-04,  3.29503094e-04,  1.19024147e-03,
+            3.92011136e-04,  9.62169025e-04,  2.72527819e-04,  9.41814213e-04,
+            1.09355549e-03, -1.26423276e-03,  1.09647701e-03,  2.82808337e-04,
+            6.38054338e-05, -3.27907803e-04, -1.91306815e-03, -2.22220727e-03,
+           -5.20148544e-04, -4.85500468e-04,  1.82340166e-04,  1.09674573e-03,
+           -5.63995348e-01])
+    assert np.allclose(wl_stats['wl_mean_peryear'].values, wl_mean_peryear_expected)
+    assert np.allclose(wl_stats['wl_mean_permonth'].values, wl_mean_permonth_expected)
+
+
+@pytest.mark.unittest
+def test_calc_wltidalindicators(prediction_extremes):
+    ext_stats = kw.calc_HWLWtidalindicators(prediction_extremes)
+    ext_stats_tzone = kw.calc_HWLWtidalindicators(prediction_extremes.tz_localize("UTC+01:00"))
+    expected_keys = ['HW_mean', 'LW_mean', 
+                     'HW_mean_peryear', 'LW_mean_peryear', 
+                     'HW_monthmax_permonth', 'LW_monthmin_permonth', 
+                     'HW_monthmax_mean_peryear', 'LW_monthmin_mean_peryear', 
+                     'HW_monthmin_mean_peryear', 'LW_monthmax_mean_peryear']
+    for key in expected_keys:
+        assert key in ext_stats.keys()
+        assert (ext_stats[key] == ext_stats_tzone[key]).all()
+    
+    assert np.isclose(ext_stats['HW_mean'], 1.4680606974545858)
+    assert np.isclose(ext_stats['LW_mean'],-0.8534702751276967)
+    
+    hw_mean_peryear_expected = np.array([1.50039216, 1.47449849, 1.45577267, 1.44152448])
+    lw_mean_peryear_expected = np.array([-0.86997608, -0.85709345, -0.84652208, -0.84024267])
+    assert np.allclose(ext_stats['HW_mean_peryear'].values, hw_mean_peryear_expected)
+    assert np.allclose(ext_stats['LW_mean_peryear'].values, lw_mean_peryear_expected)
+
+    hw_monthmax_permonth_expected = np.array([1.70887357, 1.70693041, 1.70448206, 1.70152976, 1.69961557,
+       1.69838487, 1.69720287, 1.69551568, 1.69332416, 1.69062942,
+       1.68743283, 1.68434737, 1.6826634 , 1.6817192 , 1.68041369,
+       1.67860467, 1.67629303, 1.67347995, 1.67016685, 1.6691834 ,
+       1.66816725, 1.66664809, 1.66471394, 1.66280594, 1.66131054,
+       1.65951663, 1.65722138, 1.6549959 , 1.65389291, 1.65228867,
+       1.65079986, 1.64923816, 1.64786607, 1.64599259, 1.6437117 ,
+       1.6430369 , 1.64242114, 1.64186457, 1.6408059 , 1.63924574,
+       1.63718491, 1.63493972, 1.63476655, 1.63452784, 1.63386586,
+       1.63324442, 1.63212122, 1.63049693])
+    lw_monthmin_permonth_expected = np.array([-0.99242254, -0.99061194, -0.9894141 , -0.98883749, -0.98797913,
+           -0.98711698, -0.98601125, -0.98473314, -0.98292192, -0.98120697,
+           -0.9807385 , -0.97999275, -0.97917249, -0.97818554, -0.97691195,
+           -0.97590032, -0.97466971, -0.97314856, -0.972554  , -0.97224475,
+           -0.97166419, -0.97080684, -0.96966737, -0.96824058, -0.96617301,
+           -0.9655175 , -0.96534086, -0.96489778, -0.96418269, -0.96319016,
+           -0.96191492, -0.96035182, -0.95979141, -0.95945245, -0.95913155,
+           -0.95857315, -0.95774217, -0.95682775, -0.95604764, -0.95499088,
+           -0.95463796, -0.95469814, -0.95457444, -0.95353554, -0.95271051,
+           -0.95211143, -0.95124064, -0.95116319])
+    assert np.allclose(ext_stats['HW_monthmax_permonth'].values, hw_monthmax_permonth_expected)
+    assert np.allclose(ext_stats['LW_monthmin_permonth'].values, lw_monthmin_permonth_expected)
+    
+    hw_monthmax_mean_peryear_expected = np.array([1.69735572, 1.67290495, 1.65165594, 1.6362904 ])
+    lw_monthmin_mean_peryear_expected = np.array([-0.98599889, -0.97359719, -0.96237644, -0.95419002])
+    hw_monthmin_mean_peryear_expected = np.array([1.29661114, 1.27231033, 1.2516437 , 1.23561901])
+    lw_monthmax_mean_peryear_expected = np.array([-0.67207461, -0.6593444 , -0.64799771, -0.6390193 ])
+    assert np.allclose(ext_stats['HW_monthmax_mean_peryear'].values, hw_monthmax_mean_peryear_expected)
+    assert np.allclose(ext_stats['LW_monthmin_mean_peryear'].values, lw_monthmin_mean_peryear_expected)
+    assert np.allclose(ext_stats['HW_monthmin_mean_peryear'].values, hw_monthmin_mean_peryear_expected)
+    assert np.allclose(ext_stats['LW_monthmax_mean_peryear'].values, lw_monthmax_mean_peryear_expected)