Releasing low res the same day or keeping it slow, but quality

doublevr

From what you said here it sounds like you can scale horizontally by adding more servers:

doublevr But sometimes, it happens that we receive several hundred encoding tasks within a few hours and it makes everything slower than usual, but most of the time we have enough capacity to process videos at a usual speed.

If you have multiple encoding jobs running simultaneously on the same machine then it would probably go quicker if you do one at a time and queue the others.