SukkoPera:
Indeed, it is. I have a signal that I must sample on a 12 us clock. Of course I use an interrupt: it seems it takes about 3 us to enter the handler and 2 more to leave it. If running at 8 MHz, like I plan, that doubles and doesn't leave much time for the actual job :(.
The benchmark sticky you quoted is not perfect but I figured your normally going to want to act on result from reading so the for-next loop overhead sort of makes up for this.
To sample at that speed I would consider a faster MCU/clock speed as you won't have much time to do anything else. If you supply more details of what you need/want then maybe we can suggest options.