Very slow opening MKV with VLC 3.x and also dev v4.0

Loading MKVs takes 18 seconds. Other file types open in 0-3 seconds.

-=Problem does not exist in VLC portable 2.1.3=-

Tried:

1. VLC reinstall + delete preferences.
2. Windows 10 clean reinstall(!)
3. delete plugins.dat
4. set cache to 1,000 ms
5. disable "look for plugins on startup"
6. Disabled all Win10 App Protection features.
7. disabled hardware acceleration.

OS: Windows 10 education, build 19041.388 BUT it started at least 6 months ago, so it affects previous builds.

Please inform me if more info is required