I tried the blink program with no delay, ( assuming that this is as quick and simple as it can get ) and the switched output was 8uS period, and with DigitalWriteFast it was 860 nS or 1.16 Mhz, so I guess I will shelve this idea for now.
I had :-
int ledPin = 13; // LED connected to digital pin 13
void setup() {
pinMode(ledPin, OUTPUT);
}
void loop()
{
digitalWriteFast(13, HIGH); // set the LED on
digitalWriteFast(13, LOW); // set the LED off
}