Android 9, VLC stable of 15 October 2021.
I have several large ( even > 20h ) .ogg files.
Short ones, and long .mp3's start playing immediately.
Long .ogg's take a few seconds per hour lenght to start and to go through big time jumps.
After that, reproduction is stable.
Issue seems to be related to current version only.
Not a show stopper esp. considering big files are for ambience only. But I'm curious as to the mechanics!