Correction, the vast majority of users will not be able to explain that they feel a responsiveness issue, but they do indeed feel it. Less hardcore gamers will often just blame other things -- loose controls, or ignored button presses, or what not.
I see it pretty regularly in playtests and such. Anyway, it's not something someone can simply say "Hey, this app took longer than 50ms to respond!" It's more of a gut feeling that something about it is slow.
They are likely aiming at the 50ms threshold because that's the point at which you really can't notice it, most of the time. At > 50ms, it becomes more and more noticeable, and by just 100ms people actively start complaining about the slowness of response.
edit:
As for the FPS gaming issue, the reality is that you don't notice it in FPS gaming because the games are built to hide it from you -- when you press your fire button, your gun fires, regardless of net latency. And that has been true for a very long time. I think the last game to really wait for server response before doing anything was Quake 3, and probably some of the Quake 3 engine games.
I could never stand Quake 3 for that reason, if you had anything higher than 30-40ms it started feeling like you weren't controlling the game.
Old hardcore quake3 player here. Q3 didn't wait for a server response, but the net-code was not that great and the interpolation did iirc not really take into account server responses. You could end up with players teleporting because of network issues. OSP was the first mod iirc that tried to fix it and it was somewhat successful.
The one mod that made great strides to fix the issues with the Q3 net-code was CPM (www.promode.org). 50-70ms to the server felt like 20-30ms in Vanilla Q3 and when you hit 20-30ms in CPM it felt like LAN play in VQ3. It fixed the niggling issues of lost packages (caused players to warp) as well. You even had some players that intentionally downloaded things in the background to cause dropped/delayed packets so that they would warp.
Eventually the net-code in CPM got so good the community even had cross-Atlantic competitions. The team I played in had 100-120 ping vs west-coast American teams on NYC servers and it was actually playable to the point where you could have fairly fair fights with them. Except against Team Abuse... :p. Man what a schooling in team-play and lock-downs of maps they gave my team.
"Correction, the vast majority of users will not be able to explain that they feel a responsiveness issue, but they do indeed feel it."
That sits well with me. Upvoted! It is true that "clunkiness" is a valid complain, but i've always attributed it to a combination of all latency-inducing factors.
I see it pretty regularly in playtests and such. Anyway, it's not something someone can simply say "Hey, this app took longer than 50ms to respond!" It's more of a gut feeling that something about it is slow.
They are likely aiming at the 50ms threshold because that's the point at which you really can't notice it, most of the time. At > 50ms, it becomes more and more noticeable, and by just 100ms people actively start complaining about the slowness of response.
edit:
As for the FPS gaming issue, the reality is that you don't notice it in FPS gaming because the games are built to hide it from you -- when you press your fire button, your gun fires, regardless of net latency. And that has been true for a very long time. I think the last game to really wait for server response before doing anything was Quake 3, and probably some of the Quake 3 engine games.
I could never stand Quake 3 for that reason, if you had anything higher than 30-40ms it started feeling like you weren't controlling the game.