diff --git a/.github/workflows/linux-wpt.yml b/.github/workflows/linux-wpt.yml
index 79d7a6aabe6..063efc6eb53 100644
--- a/.github/workflows/linux-wpt.yml
+++ b/.github/workflows/linux-wpt.yml
@@ -81,7 +81,7 @@ jobs:
           ./mach test-wpt \
             $WPT_ALWAYS_SUCCEED_ARG \
             --product servodriver --headless \
-            --${{ inputs.profile }} --processes $(nproc) --timeout-multiplier 2 \
+            --${{ inputs.profile }} --processes 1 --timeout-multiplier 2 \
             --total-chunks ${{ inputs.number-of-wpt-chunks }} --this-chunk ${{ matrix.chunk_id }} \
             --log-raw wpt-full-logs/linux/raw/${{ matrix.chunk_id }}.log \
             --log-wptreport wpt-full-logs/linux/wptreport/${{ matrix.chunk_id }}.json \
diff --git a/.github/workflows/mac-wpt.yml b/.github/workflows/mac-wpt.yml
index caa5bdb0f89..9dd09648552 100644
--- a/.github/workflows/mac-wpt.yml
+++ b/.github/workflows/mac-wpt.yml
@@ -52,7 +52,7 @@ jobs:
           mkdir -p wpt-full-logs/macos
           ./mach test-wpt \
             --product servodriver --headless \
-            --${{ inputs.profile }} --processes $(sysctl -n hw.logicalcpu) --timeout-multiplier 8 \
+            --${{ inputs.profile }} --processes 1 --timeout-multiplier 8 \
             --total-chunks ${{ env.max_chunk_id }} --this-chunk ${{ matrix.chunk_id }} \
             --log-raw wpt-full-logs/macos/${{ matrix.chunk_id }}.log \
             --log-raw-unexpected wpt-filtered-logs/macos/${{ matrix.chunk_id }}.log \