A preset controls how much computational effort the encoder spends looking for
efficient ways to represent each frame. x264 and x265 both offer the same ladder,
from ultrafast through medium to veryslow and placebo.
The important property: at a fixed CRF, a slower preset produces a smaller file at the same quality. It is not a quality setting — CRF is the quality setting — it is an efficiency setting. You are trading your time for the encoder's thoroughness.
Rough shape of the trade, on H.264:
- Each step slower costs meaningfully more time, roughly doubling by the lower end of the ladder.
- The size saving per step diminishes. The gap between
mediumandslowis worth having; the gap betweenveryslowandplacebois not worth the name. ultrafastis dramatically larger for the same quality, and exists for live encoding where there is no choice.
medium is the default in x264 and is what the measurements on this site were
produced with. For a file you are compressing once and sending, slow is
usually a sensible upgrade: a few percent smaller for a few extra minutes.
One thing presets do not interact with the way people expect: a slower preset will not rescue footage from too low a bitrate. It makes the bits go further. It does not create any.