The problem people often run in to when persisting the nix store is that it can be very large, and updates often. This adds a significant tax on setup/teardown time and sometimes storage costs. The completely free and unsubsidized Magic Nix Cache's design alleviates that on GitHub Actions by persisting each path individually in the actions cache. FlakeHub Cache brings a for-cost component to that, which exposes the cache to your workstation and production infrastructure.

Yes, this is exactly the problem I hit when I try to cache the Nix store on CircleCI.

To clarify I meant having /nix/store on some kind of persistent volume rather than copying it at the start and end of every job, that indeed would be painfully slow and extremely inefficient. I don't know if this is possible with CircleCI.