Frame time
Also called frame interval · frametime
Frame time is the gap between one frame and the next, measured in milliseconds. At 60Hz each frame lasts about 16.7ms and at 144Hz about 6.9ms.
The refresh rate test samples frame intervals for two seconds and reports how much they vary as well as their average.
In practice
Average frame rate hides problems that frame time shows. A game can average 60 frames per second while some frames take twice as long as others, which looks like stutter. Smooth motion depends on frames arriving evenly. That is why frame time graphs are used to judge smoothness rather than average frames per second alone.
How to check
The refresh rate test samples frame intervals for two seconds and reports both the average and how much they vary. Low variation on a fixed refresh screen means steady delivery. High variation usually means variable refresh is active, which is expected. Close other heavy tabs before testing, since they can delay frames.