From ZDoom Wiki
Fast monsters can be obtained in several ways:
- With a skill setting containing FastMonsters
- With sv_fastmonsters console variable, which can be set with the -fast parameter or the DM flag 32768
- By using +ALWAYSFAST in a monster definition
Fast monsters can be prevented by using the +NEVERFAST in a monster definition.
In fast monsters mode, the following effects apply:
- Actors with fast states halve the tic duration of these states
- Actors use their FastSpeed, if defined, instead of their normal Speed
- If nightmarefast is set in GameInfo, states calling A_Chase (or a variant) and having a duration greater than three tics have their duration halved to a minimum of three. This effect can be obtained independently of fast monsters mode by using the relevant flag in the A_Chase call.