> execute instructions faster (via -O3)

One problem with -O3 is that it disregards any effects of the cache (and yes, ESP32s have a bit of cache memory). I recommend using -Os as the default optimization level, as it often has the same or almost the same performance benefits as -O3. Less cache pressure is beneficial, especially for bigger applications, and the smaller code size is of course nice on embedded systems that have limited amounts of flash storage. If you know some specific functions or source files are performance sensitive, you can use pragmas to change optimization for those:

    #pragma GCC optimize("O3")

See https://gcc.gnu.org/onlinedocs/gcc/Function-Specific-Option-.... Clang has a similar pragma, see https://clang.llvm.org/docs/LanguageExtensions.html#extensio....