It's been a while, but here what happened. I think I wrote that I'll make a video of the said problem but just didn't care enough to delve into this any deeper. Used the different plugin with same functionality, but that matters little. It's RPGM mv problem and using these types of plugins will alienate a big portion of players.
The same thing happens with YSP_player.
I hope this won't be seen as attacking plugin maker, I encountered and confirmed this problem. Just want to report it with actual video proof.
Currently tested with Japanese plugin which plays WebM, ysp and PKD player. All had same issue.
Tested it myself with a fresh project, one plugin, one event to play the command, other players were also affected. Mostly it happens on low-mid range pc.
I think 4gb ram, hd5500, i5-5200u should be more than enough to play WebM short loop on an empty map, but I think one player of mine reported the same problem playing on PC gaming desktop with 1080TI.
I have no idea why RPGM does this, I just wanted to report a problem with actual video this time and example how it looks.
While this video was captured using a different plugin to play WebM, the issue and behavior are the same.