[HELP] after encoding ass into mp4, but there is no subtitle at all

[Selur]

if other player also have the FPS drop problem, how to solve?

Find out what is wrong with the file.
You partially did this, as you noticed that not using chapters fixes the problem.
So either there is a problem with PotPlayers mp4 support or a problem with MP4Box creating the files.
Since I have no such problem in MPC-HC with any of the mp4 files I create here with chapters on any of my devices or players, I suspect that it's a problem of PotPlayer.

i hope you could research on it.

Not happening.

Cu Selur