Note that missing the 16 ms barrier doesn’t mean that the app can hit 17ms and thus achieve a slightly smaller rate like 59 fps. Buffers can only be posted to the screen at 1/60th of a second intervals. So if your app misses the window for one frame, it will wait until the next. So if it takes your app 17 ms to render a frame, this will be visible to the user as taking twice as long, or only hitting 30 fps for that frame.

Can anyone explain it more cleary for me? Thanks