I found this ffmpeg: https://github.com/ffmpegwasm/ffmpeg.wasm
It appears to be an updated version which actually works for my x265 videos.
The current one doesn't seem to like x265 10-bit

However, when I send my .ts files to that ffmpeg implementation, the decoding is at about 7x on a i5-7300HQ.
Is there a way to update the ffmpeg in use by h265player to that ffmpeg?