[eqvinox]

It doesn't become subjective; things in -O3 can objectively be understood to produce equal or faster code for a higher build cost in the vast majority of cases, roughly averaged across platforms. (Without loss in correctness.)

If you know your exact target and details about your input expectations, of course you can optimize further, which might involve turning off some things in -O3 (or even -O2). On a whole bunch of systems, -Os can be faster than -O3 due to I-cache size limits. But at-large, you can expect -O3 to be faster.

Similar considerations apply for LTO and PGO. LTO is commonly default for release builds these days, it just costs a whole lot of compile time. PGO is done when possible (i.e. known majority inputs).