diff --git a/nixbld-etc-nixos/configuration.nix b/nixbld-etc-nixos/configuration.nix
index d9c64903..ec1bab29 100644
--- a/nixbld-etc-nixos/configuration.nix
+++ b/nixbld-etc-nixos/configuration.nix
@@ -575,11 +575,11 @@ in
   boot.kernel.sysctl."kernel.dmesg_restrict" = true;
   services.udev.packages = [ pkgs.sane-backends ];
 
-  nix.settings.max-jobs = 10;
+  nix.settings.max-jobs = 4;
+  nix.settings.cores = 8;
   nix.nrBuildUsers = 64;
   services.hydra = {
     enable = true;
-    useSubstitutes = true;
     hydraURL = "https://nixbld.m-labs.hk";
     notificationSender = "hydra@m-labs.hk";
     minimumDiskFree = 15;  # in GB
@@ -693,6 +693,7 @@ in
     experimental-features = nix-command flakes
   '';
   nix.settings.extra-sandbox-paths = ["/opt"];
+  nix.settings.substituters = pkgs.lib.mkForce [];
 
   services.mlabs-backup.enable = true;
   services.ghbackup.enable = true;