host: enable simultaneous multithreading for headless

nixos/shared/headless.nix

@@ -5,7 +5,11 @@
     "${inputs.nixpkgs}/nixos/modules/profiles/headless.nix"
   ];
 
+  # Fix for GLIBC errors due to 'scudo' from hardened profile.
+  # https://github.com/NixOS/nix/issues/6563
   environment.memoryAllocator.provider = "libc";
+  # Disabled by hardened profile, big performance hit.
+  security.allowSimultaneousMultithreading = true;
 
   boot.tmp.cleanOnBoot = true;
   zramSwap.enable = true;