Build and use a disk image for the Nix store, instead of accessing the host's one through 9p.
For applications which do a lot of reads from the store, this can drastically improve performance, but at the cost of disk space and image build time.
As an alternative, you can use a bootloader which will provide you with a full NixOS system image containing a Nix store and avoid mounting the host nix store through virtualisation.mountHostNixStore.