IRAM_ATTR
void IRAM_ATTR test_interrupt()
By flagging a piece of code with the IRAM_ATTR attribute we are declaring that the compiled code will be placed in the Internal RAM (IRAM) of the ESP32.
Otherwise the code is placed in the Flash. And flash on the ESP32 is much slower than internal RAM.
If the code we want to run is an interrupt service routine (ISR), we generally want to execute it as quickly as possible. If we had to ‘wait’ for an ISR to load from flash, things would go horribly wrong.