[MarekKnapek]

My idea is: 1) Determine how frequently you want to update the number on screen. Every frame is too often, once per second is too infrequent. Maybe 4× per second is OK. 2) Determine how much time to average over. One or two seconds seems fine. 3) From those two numbers, compute number of buckets. For example update 4× per second, average over 2 seconds, gives you 8 buckets. 4) Every time a frame is drawn, based on current time, you either a) increment the counter in the last bucket or b) shift all buckets over one position and increment the new last bucket from zero to one. 5) Average the sum of frames drawn over that time period you decided earlier. 6) Don't display fraction of a FPS. 7) Done. It costs you 8 integers, plus maybe one more for a timestamp of the last bucket, plus maybe one more to implement ring buffer instead of shifting all those integers over. Less than a single CPU cache line.