diff --git a/pkgs/servers/matrix-synapse/default.nix b/pkgs/servers/matrix-synapse/default.nix
index 74d27cb29521..5f48ee51e79c 100644
--- a/pkgs/servers/matrix-synapse/default.nix
+++ b/pkgs/servers/matrix-synapse/default.nix
@@ -86,13 +86,13 @@ buildPythonApplication rec {
 
   doCheck = !stdenv.isDarwin;
 
-  checkPhase = ''
+  checkPhase = let testFlags = lib.optionalString (!stdenv.isAarch64) "-j $NIX_BUILD_CORES"; in ''
     runHook preCheck
 
     # remove src module, so tests use the installed module instead
     rm -rf ./synapse
 
-    PYTHONPATH=".:$PYTHONPATH" ${python3.interpreter} -m twisted.trial -j $NIX_BUILD_CORES tests
+    PYTHONPATH=".:$PYTHONPATH" ${python3.interpreter} -m twisted.trial ${testFlags} tests
 
     runHook postCheck
   '';