See https://github.com/cisco/openh264 as an alternative to the ffmpeg based decoder. Limitations (from what I can see on their home page):
commits seem to have dried up, I am betting on vp9 and AV1 (#1668)
This is the H.264 de-/encoder that is available on e.g. Fedora and RHEL, where x264 is not available.
The performance of this codec is nowhere near enough for us to use.
Our repository provides packages with the x264 encoder and the ffmpeg decoder for both centos (RHEL) and Fedora.
this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/721