Why subtitles are so troublesome on jellyfin?

[bitmap]

Need mediainfo or ffprobe of the media file. As somebody already said, it's likely a graphics-based subtitle format. These formats require transcoding with a ton of clients (not all). In addition, other information (OS of your install, virtualization or host installed, version numbers of server and apps) would help to investigate as well.

As a counterpoint to your specs, I run a headless Linux box (Ubuntu 22.04 LTS) with a non-compliant kernel that by all accounts shouldn't be able to use the full extent of the iGPU and I can watch a high-bitrate remux with PGS subs on that require transcoding with very little delay. There is a "restart" as the transcode job begins, but it's in the realm of less than five seconds before playback resumes. I have no additional hardware, just standard QuickSync enabled with a some hacked-together drivers since I didn't want to upgrade my OS.