This is a benchmark to test the input response time. The experiment is done with the iPhone app "Is it snappy?", and thus end-to-end (photons to photons, anyway)
The latency is measured by recording the time between a j keystroke on a keyboard with right index finger, and the first frame that a sign of updated buffer is reflected on my screen.
The key frames are picked manually by stepping through the single frames.
My iPhone7 is capable of 120fps recording, and thus ~8.33ms per frame.
instance time frames
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nvim-qt 75.0ms 9
nvim-qt 58.3ms 7
conhost 83.3ms 10
fvim 100.0ms 12
fvim 116.7ms 14
fvim 116.7ms 14
nvim-gtk 83.3ms 10
Looks like fvim is constantly lagging behind for a few frames , and in the worst case, 2x less responsive than nvim-qt.
The tests are done on Windows10, with immediate renderer (and thus better latency) -- this means that the readings on X11 will be even worse.