I don't know how Terraform works, but `virt-builder` will cache the xz-compressed template images locally on your first pull. So for example, if you build a Fedora image:

    $ virt-builder fedora-32 --size 10G --format qcow2

Then the (xz-compressed) template Fedora 32 image will be cached under ~/.cache/virt-builder. So your subsequent Fedora 32 image provisioning will be much faster.

You might want to give it a whirl and see if it satisfies your needs; `virt-builder` should be available on most major Linux distributions.