diff --git a/.github/workflows/ci-julia-nightly.yml b/.github/workflows/ci-julia-nightly.yml
index 94e3854..d018ed3 100644
--- a/.github/workflows/ci-julia-nightly.yml
+++ b/.github/workflows/ci-julia-nightly.yml
@@ -4,6 +4,14 @@ on:
     branches: [master, main]
     tags: ["*"]
   pull_request:
+
+concurrency:
+  # group by workflow and ref; the last slightly strange component ensures that for pull
+  # requests, we limit to 1 concurrent job, but for the master branch we don't
+  group: ${{ github.workflow }}-${{ github.ref }}-${{ (github.ref != 'refs/heads/master' && github.ref != 'refs/heads/main') || github.run_number }}
+  # Cancel intermediate builds, but only if it is a pull request build.
+  cancel-in-progress: ${{ startsWith(github.ref, 'refs/pull/') }}
+
 env:
   PYTHON: ~
 jobs:
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 66b1abb..cf751e1 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -4,6 +4,14 @@ on:
     branches: [master, main]
     tags: ["*"]
   pull_request:
+
+concurrency:
+  # group by workflow and ref; the last slightly strange component ensures that for pull
+  # requests, we limit to 1 concurrent job, but for the master branch we don't
+  group: ${{ github.workflow }}-${{ github.ref }}-${{ (github.ref != 'refs/heads/master' && github.ref != 'refs/heads/main') || github.run_number }}
+  # Cancel intermediate builds, but only if it is a pull request build.
+  cancel-in-progress: ${{ startsWith(github.ref, 'refs/pull/') }}
+
 env:
   PYTHON: ~
 jobs:
@@ -18,12 +26,24 @@ jobs:
           - '1.10'
         os:
           - ubuntu-latest
-          - windows-latest
-          - macOS-latest
         threads:
-          - '2'
+          - '1'
+          - '5'
         arch:
           - x64
+        include:
+          - arch: aarch64
+            os: macos-latest
+            version: '1'
+            threads: '1'
+          - arch: x64
+            os: macos-latest
+            version: '1'
+            threads: '1'
+          - arch: x64
+            os: windows-latest
+            version: '1'
+            threads: '1'
     steps:
       - uses: actions/checkout@v4
       - uses: julia-actions/setup-julia@v1