You have to profile for your specific use case. Some programs run slower under O3 because it inlines/unrolls more aggressively, increasing code size (which can be cache-unfriendly).

Yeah, -O3 generally performs well in small benchmarks because of aggressive loop unrolling and inlining. But in large programs that face icache pressure, it can end up being slower. Sometimes -Os is even better for the same reason, but -O2 is usually a better default.