diff --git a/src/m3ninx/index/segment/builder/builder.go b/src/m3ninx/index/segment/builder/builder.go
index e01d4846ab..2ae2a4a5e3 100644
--- a/src/m3ninx/index/segment/builder/builder.go
+++ b/src/m3ninx/index/segment/builder/builder.go
@@ -55,8 +55,6 @@ type indexJob struct {
 }
 
 type builder struct {
-	sync.Mutex
-
 	opts      Options
 	newUUIDFn util.NewUUIDFn
 
@@ -65,7 +63,7 @@ type builder struct {
 	batchSizeOne index.Batch
 	docs         []doc.Document
 	idSet        *IDsMap
-	fields       *fieldsMap
+	fields       *shardedFieldsMap
 	uniqueFields [][][]byte
 
 	wg          sync.WaitGroup
@@ -87,9 +85,6 @@ func NewBuilderFromDocuments(opts Options) (segment.CloseableDocumentsBuilder, e
 		idSet: NewIDsMap(IDsMapOptions{
 			InitialSize: opts.InitialCapacity(),
 		}),
-		fields: newFieldsMap(fieldsMapOptions{
-			InitialSize: opts.InitialCapacity(),
-		}),
 		uniqueFields: make([][][]byte, 0, concurrency),
 		indexQueues:  make([]chan indexJob, 0, concurrency),
 		closed:       atomic.NewBool(false),
@@ -107,6 +102,7 @@ func NewBuilderFromDocuments(opts Options) (segment.CloseableDocumentsBuilder, e
 		}
 		shardUniqueFields := make([][]byte, 0, shardInitialCapcity)
 		b.uniqueFields = append(b.uniqueFields, shardUniqueFields)
+		b.fields = newShardedFieldsMap(concurrency, shardInitialCapcity)
 	}
 
 	return b, nil
@@ -126,9 +122,7 @@ func (b *builder) Reset(offset postings.ID) {
 	b.idSet.Reset()
 
 	// Keep fields around, just reset the terms set for each one.
-	for _, entry := range b.fields.Iter() {
-		entry.Value().reset()
-	}
+	b.fields.ResetTermsSets()
 
 	// Reset the unique fields slice
 	for i, shardUniqueFields := range b.uniqueFields {
@@ -236,19 +230,14 @@ func (b *builder) index(
 
 func (b *builder) indexWorker(indexQueue chan indexJob) {
 	for job := range indexQueue {
-		terms, ok := b.fields.Get(job.field.Name)
+		terms, ok := b.fields.ShardedGet(job.shard, job.field.Name)
 		if !ok {
-			b.Lock()
 			// NB(bodu): Check again within the lock to make sure we aren't making concurrent map writes.
-			terms, ok = b.fields.Get(job.field.Name)
-			if !ok {
-				terms = newTerms(b.opts)
-				b.fields.SetUnsafe(job.field.Name, terms, fieldsMapSetUnsafeOptions{
-					NoCopyKey:     true,
-					NoFinalizeKey: true,
-				})
-			}
-			b.Unlock()
+			terms = newTerms(b.opts)
+			b.fields.ShardedSetUnsafe(job.shard, job.field.Name, terms, fieldsMapSetUnsafeOptions{
+				NoCopyKey:     true,
+				NoFinalizeKey: true,
+			})
 		}
 
 		// If empty field, track insertion of this key into the fields
diff --git a/src/m3ninx/index/segment/builder/sharded_fields_map.go b/src/m3ninx/index/segment/builder/sharded_fields_map.go
new file mode 100644
index 0000000000..f7f3528555
--- /dev/null
+++ b/src/m3ninx/index/segment/builder/sharded_fields_map.go
@@ -0,0 +1,55 @@
+package builder
+
+type shardedFieldsMap struct {
+	data []*fieldsMap
+}
+
+func newShardedFieldsMap(
+	numShards int,
+	shardInitialCapacity int,
+) *shardedFieldsMap {
+	data := make([]*fieldsMap, 0, numShards)
+	for i := 0; i < numShards; i++ {
+		data = append(data, newFieldsMap(fieldsMapOptions{
+			InitialSize: shardInitialCapacity,
+		}))
+	}
+	return &shardedFieldsMap{
+		data: data,
+	}
+}
+
+func (s *shardedFieldsMap) Get(k []byte) (*terms, bool) {
+	for _, fieldMap := range s.data {
+		t, found := fieldMap.Get(k)
+		if found {
+			return t, found
+		}
+	}
+	return nil, false
+}
+
+func (s *shardedFieldsMap) ShardedGet(
+	shard int,
+	k []byte,
+) (*terms, bool) {
+	return s.data[shard].Get(k)
+}
+
+func (s *shardedFieldsMap) ShardedSetUnsafe(
+	shard int,
+	k []byte,
+	v *terms,
+	opts fieldsMapSetUnsafeOptions,
+) {
+	s.data[shard].SetUnsafe(k, v, opts)
+}
+
+// ResetTerms keeps fields around but resets the terms set for each one.
+func (s *shardedFieldsMap) ResetTermsSets() {
+	for _, fieldMap := range s.data {
+		for _, entry := range fieldMap.Iter() {
+			entry.Value().reset()
+		}
+	}
+}