A refresh rate of 60 FPS is typical for any device these days. This means that in the timeframe of 1 second, the browser has to draw 1 frame on the screen, which in turn means that each frame has a time budget of 16.6ms (1 second / 60 frames = 16.66ms). Drawing this frame in 16.6ms includes:

• Executing JS
• Calculating styles
• Calculating layout
• Painting pixels on layers
• Compositing those layers

Since all of these tasks run in a single thread, when they take more than ~10ms (not 16ms, because there is also other work being done by the browser), frames start to drop. Drop enough frames and visually perceptible lagginess starts to occur.

Sources: Rendering Performance by Paul Lewis