IIUC Magic Nix Cache uses Github Actions Cache, which means there is waiting for the cache contents to be loaded onto the runner instance. This can in no way be as fast as what namespace.so does, which is mounting a disk image with /nix/store.
I'm going to have to look into this mounting option. I wanted to try making volumes very similar to this for reusing existing /target directories for internal company Rust builds. Sounds like Namespace could make this very easy to try out.
Just need to figure out what the cache size limitations are..
To be clear, I would not expect it to be as fast! But I do wonder what the percentage drop-off would be and, if small enough, if that would make staying on Actions more compelling than a bespoke solution.