diff --git a/.github/workflows/haskell.yml b/.github/workflows/haskell.yml
index 0659eeda1..0f0b51c4c 100644
--- a/.github/workflows/haskell.yml
+++ b/.github/workflows/haskell.yml
@@ -120,7 +120,7 @@ jobs:
     # this should be a no-op, but if the cache key was not found we need to build stuff so we can
     # cache it for the next step.
     - name: Install dependencies
-      run: cabal build all --enable-tests --only-dependencies -j --ghc-option=-j4
+      run: cabal build all --enable-tests --only-dependencies
 
     # Always store the cabal cache.
     - name: Cache Cabal store
@@ -141,7 +141,7 @@ jobs:
         TMPDIR: ${{ runner.temp }}
         TMP: ${{ runner.temp }}
         KEEP_WORKSPACE: 1
-      run: cabal test all --enable-tests --test-show-details=direct -j1
+      run: cabal test all --enable-tests --test-show-details=direct
 
     # Uncomment the following back in for debugging. Remember to launch a `pwsh` from
     # the tmux session to debug `pwsh` issues. And be reminded that the `/msys2` and
diff --git a/cabal.project b/cabal.project
index 8a4ac9d9e..ad862869f 100644
--- a/cabal.project
+++ b/cabal.project
@@ -39,12 +39,15 @@ package bitvec
    flags: -simd
 
 tests: True
-
 test-show-details: direct
 
 -- Always write GHC env files, because they are needed for ghci.
 write-ghc-environment-files: always
 
+-- enable parallel builds
+jobs: $ncpus
+semaphore: True
+
 -- IMPORTANT
 -- Do NOT add more source-repository-package stanzas here unless they are strictly
 -- temporary! Please read the section in CONTRIBUTING about updating dependencies.