LLVM and GCC are so close together these days that it makes almost no difference practically, and when it does you should start optimizing based around your code not your compiler.
Root of all evil etc. etc.
Root of all evil etc. etc.
Your comments read like you're explaining optimization to a beginner, it's a bit bad faith tbh.
GCC and LLVM have broadly similar performance characteristics(i.e. for any arch/uBenchmark combo there is another that puts the other one faster).
If GCC is appreciably faster for your purpose, then fair enough but LLVM is not drastically slow by any means (Especially when you consider that any "Anything you can do..." Between LLVM and GCC moves much faster on the LLVM side due to a much saner codebase)