It's not that compilation itself was slow with Guix, but (to my memory) it was because all Guix packages are completely separate by design and have dependencies which are not shared, so you might have to compile, say, five different versions of guile to satisfy those five different dependencies in five different packages.
Compiling guile from scratch was, by the way, insanely slow, and because I struggled to get Guix working right on my first try, I had to compile it over and over again, which took about a day each time. A "guix pull" took four hours on my system, and I had to do it twice each time, once for root and once for my own user. My first "guix pull" took two weeks due to all the errors, toubleshooting and restarts the process took as I tried to get Guix working as it should.
In addition, it seems that Guix packages don't have optimizations enabled, which is a deal breaker for me, as optimizing the packages I install is a significant reason I compile them from scratch in the first place, as I'm running on an old, slow machine which could use all the performance boosts it can get.
Finally, I might have a broken environment on my system which makes compilation even slower still than it otherwise would be, but until I get it fixed I have to live with it, and so for all of these reasons compiling everything from scratch on Guix is just not practical for me right now.
Maybe with a newer, much faster machine I'd be ok with it.