FPS is an average: how many frames arrived in a second. Frame pacing is how evenly they arrived. A screen can show the right FPS and still stutter.
- At 60 Hz a frame is due every 16.7 ms. Sixty frames with some late and some bunched look rougher than sixty on time.
- SystemCheck's frame pacing test counts a frame as late at over 1.5 times the frame interval and as a hitch at 2.5 times or more.
- It also reports the 99th percentile interval and the 1% low, the mean of the slowest 1% of frame times as fps. There is no composite score.
The short answer
FPS tells you how many frames arrived; frame pacing tells you whether they arrived when they were due. The eye notices the late ones. That is why a game can report a steady 60 FPS and still feel uneven, and why frame time figures say more about smoothness than an average.
Why the average hides stutter
An average over a second treats one 50 ms frame followed by short ones the same as a run of even frames, as long as the count comes out the same. On screen they are not the same: the long frame is a visible pause. Percentile figures and the 1% low exist to pull those slow frames out of the average where you can see them.
How SystemCheck measures pacing
The frame pacing test times twenty seconds of frames as each one arrives and reports four figures with published lines.
- Late share: frames over 1.5 times the frame interval.
- Hitches: frames 2.5 times the interval or longer, counted once and not again as late.
- The 99th percentile frame interval, in milliseconds.
- The 1% low: the mean of the slowest 1% of frame times, as fps.
What it does not measure
It measures the browser's frames, not a game's; a game engine paces its own frames. A variable refresh rate panel changes its rate with the content, while the browser sees one fixed rate. A covered or background tab is slowed by the browser, which would read as stutter, so the tab must stay in front.
Terms used here
Questions
- Is higher FPS always smoother?
- No. Higher FPS shortens the gap between frames, but uneven delivery still shows as stutter. Even pacing at a lower rate can look smoother than uneven pacing at a higher one.
- Is frame pacing the same as input lag?
- No. End-to-end input lag (from the device to the screen) is the delay between an action and its result on screen, which a web page cannot measure; a page sees only the browser's part of that chain. Pacing is whether frames arrive evenly once they are drawn.