[Steam Deck][Latency] Noticeable input lag with frame limiter turned on

[andrebrait]

I have noticed there is some input latency when the frame limiter is turned on versus when it's off or when the game is limiting FPS internally, in The Witcher 3.

I took some "measurements" by filming a button prompt reacting to me pressing the X button. I used my Samsung Galaxy S22+ which can shoot at 960fps. It's a crude measurement, but I tried the same strategy with the UFO test on my 144Hz monitor and I got an accurate result, so I think the results here are somewhat valid.

Both options were tested with V-Sync off in the game settings. Both options were tested with a limit of 30 frames per second. The frames were counted from the moment my finger bottoms out the X button to the first panel refresh pixels start to change in the button prompt.

• In-game FPS limiter: 77 frames, which equal to around 80ms, or 4.8 panel refreshes at 60Hz
• Gamescope FPS limiter: 150 frames, which equal to around 156ms, or 9.36 panel refreshes at 60Hz

While I can't get an exact figure for the delay, it seems to be around double whatever the normal delay is. I suspect this has to do with how the strategic buffer withholding works in Gamescope.

If I understood it correctly and if we can get the time spent by the game to render the frame, it should be possible to implement a less-reliable, low-latency mode by changing how the wait is done and when the buffer is freed for the game to render the frame to. My initial thoughts are to let the user choose that, aware that a frame skip or tearing might happen if the game exceeds the target rendering time.