Slow Nix builds

I am trying to do nix builds in AWS codebuild containers. Builds are working, but builds are not utilizing cpu fully. When I do nix builds locally it takes 100% of all my cores. As AWS codebuild spawns fresh containers there is no local nix store cached. I use cachix as nix cache.And push local store to cachix at the end.

Could it be because nix builds are happening inside docker containers?