How to disable Zlib compression on a PGS subtitles
Zlib reduces the file size of a subtitle, but using it consumes more resources and causes problems on older hardware, so we will disable it in this quick guide.
Tool that you need:
MKVtoolNix : https://mkvtoolnix.download/downloads.html
Go to Preferences or Press Ctrl + P
Default values and check "Disable additional lossless compression for all track types" and it should turn off automatic compression.
Restart MKVToolNix and check if it's turned off automatically:
Thanks Simulping for helping me on this!