Debian stretch has libvpx 1.6.x, which has good VP9 support but can only use a limited number of threads based on resolution (1 or 2 for low resolutions, 4 for HD). libvpx 1.7.0's row-based multithreading option can scale out to many cores at any resolution, which will make encoding VP9 video transcodes for T63805 faster than with the current default package.
There may be backported packages already; Debian-testing appears to include libvpx 1.7.0. ffmpeg will need to be updated to a version built against libvpx 1.7.0 and recent enough to include the -row-mt option for libvpx-vp9.
Note that TimedMediaHandler also needs an update to make use of the -row-mt option, will add a task for that shortly.