I've tried bumping the number of vCPUs to 4, giving the VM under test more RAM,
and it's still very unreliable for me. The Journal says:

  Jul 06 08:54:49 amnesia tor-browser.desktop[8445]: [Child 8706, MediaPlayback #1] WARNING: Decoder=72da91792200 state=DECODING_METADATA Decode metadata failed, shutting down decoder: file /var/tmp/build/firefox-a0efd2fcd6e9/dom/media/MediaDecoderStateMachine.cpp, line 379
  Jul 06 08:54:49 amnesia tor-browser.desktop[8445]: [Child 8706, MediaPlayback #1] WARNING: Decoder=72da91792200 Decode error: NS_ERROR_DOM_MEDIA_METADATA_ERR (0x806e0006): file /var/tmp/build/firefox-a0efd2fcd6e9/dom/media/MediaDecoderStateMachine.cpp, line 3445

The previous step passes fine so it's not a matter of connecting to our website,
so I have some doubt this change will help for this specific problem, but well,
it's a good thing anyway.