Arduino for STM32

Whenever I select another board all settings reset to default, including compiler flags -0s. How to make -03 as default?
I find that AVR boards are referenced to platform.local.txt, what about nucleo?

maybe try looking in platforms.txt or boards.txt?
you could patch your copy of the core locally
just be aware, -O3 can blow the budget of flash memory and sram on your stm32, there is a reason -Os is selected