In many contexts, one should rarely pass -O2/-O3. A project that is built thousands of times during development may only be run on intensive workloads (where -O2 performance is actually a necessity) a handful of times by comparison. A dev build can usually be -O0, which can dramatically improve compilation time.
It depends. O0 turns off a few trimming optimizations and could potentially causes more information (code or DWARF) to be included in the objects, which may eventually slow down the compilation. In our large code base, we found that -O1 works best in terms of compilation speed.
