Before a substitute can be requested, Guix will have to compute a
derivation locally. In the case of Guix itself this is rather
expensive.
Once we have the derivation we can ask substitute servers if they have a
binary substitute for performing the work the derivation describes.
So, local *computation* cannot be avoided, but local building should be
avoidable for most packages — here the problem is that hydra isn’t fast
enough yet. I’ve been preparing an alternative build farm at the
institute where I work, which will hopefully soon be powerful enough to
build packages more quickly than our current Hydra does.