Fast monsters is a setting which makes monsters more aggressive.
The more common demonic monsters from the original Doom are the most affected by this mode. Demons and the related spectres move and attack twice as fast as usual. Some projectiles are sped up to 15 units per tic, making imp and cacodemon fireballs 50% faster than usual, while hell knight and baron of hell projectiles are 33% faster. These and other monsters also attack more often, usually shooting and moving very little as long as they have their target in sight, which makes hitscan enemies considerably more dangerous, and the player's fist and chainsaw much less useful.
The fast monsters setting is automatically activated when playing using Nightmare! difficulty, although in this skill level they also attack immediately on sight, while in mere fast mode a couple of steps are usually evident before their first attack occurs.
The setting can be used with any skill level using the -fast command line argument.