H5 video doesn't play when hardware acceleration occupied (Intel i915) yet no error message given

A laptop which has Intel & NVIDIA optimus. Linux x86_64 (many different distros)

When Firefox, SMPlayer (MPV backend) or Virtualbox simultaneously running, Firefox H5 video always get jammed, video looks like paused. After closing SMPlayer and VBox, Firefox H5 video plays.

That problem doesn’t exist on Windows or with optirun on Linux.

Apparently that’s an issue related to GPU video acceleration.

Unchecking Firefox’s Use hardware acceleration when available doesn’t make any difference.

A video so I can test it?
I have a laptop with optimus too (not sure of the cpu) but is old (5 years I think).