35fps is ideal for DOOM, and in many instances a 486DX2/66 can come close or meet that frame rate. When the screen gets crowded, expect some slowdown.
I like how ID Software is essentially a barometer for performance :
Commander Keen will run on an 8088/8086 but is best played on a 286.
Wolfenstein 3D runs on a 286 but is best played on a 386
DOOM runs on a 386 but is best played on a 486
Quake runs on a 386 but is best played on a 486
Quake II runs on a Pentium but is best played on a Pentium II
Quake III runs on a Pentium II but is best played on a Pentium III
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