As far as I understand it, Guix will download pre-built binaries for most packages from a cache by default, and the Guix OS distribution makes sure the x86_64 binaries for the latest package descriptions are always cached, so you should usually not have to locally build packages.
But of course you can easily tweak the default configuration of packages you install and trigger a local re-build of those packages, since changing the configuration of any package causes a cache miss.