It depends on which "Arduino" you are running it on.

On an Arduino Uno, for example, then the only thing that won't work is the SerialEvent system. Everything else that matters is interrupts.

Here is main() on the Arduino AVR core:

int main(void)
{
    init();

    initVariant();

#if defined(USBCON)
    USBDevice.attach();
#endif
    
    setup();
    
    for (;;) {
        loop();
        if (serialEventRun) serialEventRun();
    }
        
    return 0;
}

As you can see, loop() is run just the same way as your fastLoop(), just not inline, and adding in a call to serialEventRun() if it has been defined.

However, on an ESP32 or ESP8266 you will find that your watchdog times out and resets the unit. This is because it relies on code being run regularly to manage the WiFi connection that keeps "kicking the dog". Without that being run (which can be called by calling the yield() function) the watchdog times out.

It depends on which "Arduino" you are running it on.

On an Arduino Uno, for example, then the only thing that won't work is the SerialEvent system. Everything else that matters is interrupts.

Here is main() on the Arduino AVR core:

int main(void)
{
    init();

    initVariant();

#if defined(USBCON)
    USBDevice.attach();
#endif
    
    setup();
    
    for (;;) {
        loop();
        if (serialEventRun) serialEventRun();
    }
        
    return 0;
}

As you can see, loop() is run just the same way as your fastLoop(), just not inline, and adding in a call to serialEventRun() if it has been defined.

If no serial event handlers have been defined, then it won't do anything there anyway.

To save clock cycles you would get more benefit (as long as you don't want to use millis(), delay(), serial communication, etc) from disabling interrupts.

However, on an ESP32 or ESP8266 you will find that your watchdog times out and resets the unit. This is because it relies on code being run regularly to manage the WiFi connection that keeps "kicking the dog". Without that being run (which can be called by calling the yield() function) the watchdog times out.