performance optimization & inlining

Hmm, I'm very surprised about the Link Time optimisation (unless you enable it by default even without the inlining flags?)

I did choose the stuff to inline based on benchmarks on STM32, but I guess on ESP8266 the balance might be more in favour of code size?

There's actually a compiler option called inline-unit-growth I think? You might have some luck with -O3 and a low-ish value for that...