From what I found on the web, people seem to be having issues with hardware acceleration on the pi 4 running Buster (CPU does the decoding instead of GPU). Is hardware acceleration not supported? If it's not, why? Will it be supported in the next release of Raspbian? And what about the H.265 hardware decoder, is that supported or will it ever be?
The foundation has their own (downstream) version of ffmpeg with HEVC decode (!) acceleration:
HEVC acceleration in VLC is currently being beta tested : https://www.raspberrypi.org/forums/viewtopic.php?f=29&t=257395
I suppose patches for Chromium will come after that ...