I can’t tell you the answer to why you see such a stark timing difference - it’s hard to say without a MWE or access to e.g. @code_native output on those machines.
That definitely shouldn’t be the case - usually, the pacman install makes problems, due to the arch repo having had numerous issues in the past, mostly related to just using system LLVM, while julia requires certain patches that just take a while to be upstreamed…
If you have a reproducible example, I’d like to try that - I’m on arch as well, but I usually compile from source due to making PRs and the like. In my experience, AUR or self compiled has been consistently faster (not by a lot though).