Back when I still cared about saving disk space, I made a cluster of NVidia Jetson Nanos running in a docker swarm configuration [1] to compress my blu-ray rips, but honestly even when you have six computers working at once, H264 on a single computer is still often faster.
On the Jetson Nanos I was lucky to get maybe 1fps in ffmpeg using VP9. Multiply that by six boards and that's about 6fps in total; ffmpeg running x264 in software mode was getting around 11fps on a single board, not even counting using the onboard encoder chip, meaning that I was getting better performance from one board using x264 than all six using VP9.
Now obviously this is a single anecdote on specific hardware, so I'm not saying that this applies to every single case, but it's a big reason why I personally have not used VP9 for anything substantial yet.