it's not so much that gcc does anything specific so much as LLVM is just really really inefficient—they don't track compilation time at all so it's easy for releases to regress, half the stuff in there is academics implementing their PhD thesis aiming for algorithmic accuracy with little regard for efficiency, and LLVM's design itself is somewhat inefficient (multiple IRs, lots and lots of pointers in IR representation, etc)

that said this makes it an excellent testbed but compilation time will keep getting slower every release until they start caring about it

Not answering your question but in my experience reading assembly output of the two compilers the unoptimized output of Clang is atrocious while GCC is closer of what a human could have wrote. Clang seems to have to do better in optimization passes to archive similar results. Usually Clang was a faster compiler in O1 but I don’t think it’s true anymore.