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.
The Nix store image is built just-in-time right before the VM is started. Because it does not produce another derivation, the image is not cached between invocations and never lands in the store or binary cache.
If you want a full disk image with a partition table and a root filesystem instead of only a store image, enable virtualisation.useBootLoader instead.