Outcome: Potential performance improvements. We'll get done whatever we can get done in 2 hours.
https://etherpad.wikimedia.org/p/instruments_observations
( Note: Lead image performance was addressed with this patch: https://phabricator.wikimedia.org/T91328 )
If you just do this in the background, then the image "jumps" when the face is detected.
To make this work without jumping, we would have to delay drawing the image. This is probably worse.
IMO I wouldn't delay the image drawing unless it was a last resort. First I would evaluate the other performance enhancements and see if they are enough.
Lead image performance was addressed with this patch: https://phabricator.wikimedia.org/T91328