Here is a step-by-step guide on how to enable fast monsters for Doom 64 on Steam.
Step 1: Install and find your copy of Doom 64 in your Steam library
Step 2: Select the Properties tab
Step 3: Select the Set Launch Options tab
Step 4: Enter -fast into the Launch Options (-skipmovies is optional if you want to skip the intro movies)
Step 5: Launch the d*rn game and enjoy, and also die a lot probably.
By buny nerd