@Inginsub It gets worse when you consider this was for old machines that the only concern was keyboard input and not something like a mouse which is significantly more annoying when it has latency or lag. I would saw anything more than mid double digits is noticeable really.

Also since they were working with CRTs they only had two sources of noticeable latency, input delay and processing time, but now with LCDs you have to add that as well. The true latency is the sum of all of the delays in the loop from user input to a change on the screen. Since the developer doesn't have a good way to check what kind of cheap displays the user buys, worst case should be assumed and just leave enough margin.