diff --git a/pkgs/by-name/co/conan/package.nix b/pkgs/by-name/co/conan/package.nix
index 386e59d985b0..67fb019af535 100644
--- a/pkgs/by-name/co/conan/package.nix
+++ b/pkgs/by-name/co/conan/package.nix
@@ -65,11 +65,6 @@ python3.pkgs.buildPythonApplication rec {
     "conan"
   ];
 
-  pytestFlagsArray = [
-    "-n"
-    "$NIX_BUILD_CORES"
-  ];
-
   disabledTests = [
     # Tests require network access
     "TestFTP"
diff --git a/pkgs/by-name/ro/routersploit/package.nix b/pkgs/by-name/ro/routersploit/package.nix
index 3f656e8e4578..14de115541af 100644
--- a/pkgs/by-name/ro/routersploit/package.nix
+++ b/pkgs/by-name/ro/routersploit/package.nix
@@ -39,8 +39,6 @@ python3.pkgs.buildPythonApplication rec {
   ];
 
   pytestFlagsArray = [
-    "-n"
-    "$NIX_BUILD_CORES"
     # Run the same tests as upstream does in the first round
     "tests/core/"
     "tests/test_exploit_scenarios.py"
diff --git a/pkgs/development/python-modules/qcodes/default.nix b/pkgs/development/python-modules/qcodes/default.nix
index 01ae8380228c..a1c5206f4af2 100644
--- a/pkgs/development/python-modules/qcodes/default.nix
+++ b/pkgs/development/python-modules/qcodes/default.nix
@@ -148,8 +148,6 @@ buildPythonPackage rec {
 
   pytestFlagsArray = [
     "-v"
-    "-n"
-    "$NIX_BUILD_CORES"
     # Follow upstream with settings
     "-m 'not serial'"
     "--hypothesis-profile ci"