investigate the performance effects of -O2 vs -O3 , possibly -O1 for gcc

To avoid gcc bugs in the production software (such as MDEV-19709), it could be better to
compile with less aggressive optimization.

Some people state that -O2 is faster than -O3, due to more compact code (less inlining or loop unrolling) and as result less cache misses . See https://stackoverflow.com/a/19985801/547065 for example.

The task is about benchmarking, and giving recommendations. From the quality standpoint, we can assume that -O2 would have less bug, and it is better tested, since it is the optimization used most often for production software.
From performance standpoint, the right -O level needs to be measured.