Frame skip when subtitle appears

I noticed this behaviour only when playing 4K movies in case subtitles are shown. When subtitle is shown, there is noticeable frame skip or stutter in picture. The same when subtitle disappears. This is happening on cca. 50% subtitles shown, not through whole movie.
On FullHD movies, this is not the case.
Mediaplayer: H96Pro+

Is this known issue?