I'm trying to remember what the 8086 chip default was. You'd have to turn the Turbo button off to start Doom IIRC.
Being a 32-bit protected mode application, Doom wouldn't have run on an 8086. The minimum requirements stated a 386 but, in all practicality, a 486 was the minimum unless you wanted the window to be so small that you had to use a magnifying glass.