I used to run Doom 2 on my 386sx. Doom 2 required a math co-processor, which the sx didn't have, so I had to use a TSR which emulated one, slowing my CPU down even more. It probably ran at 1-2 FPS, but it was "playable" enough for me to spend countless hours playing it. Ahhh, memories...
So yeah, 20 FPS is definitely what I would call playable. ;)
Doom (or Doom 2 which had basically the same engine with just bugfixes and support for Doom 2 entities) didn't actually need an FPU. Even 386DX's or 486SX's had no FPU so no games from the pre-1995 era really depended on it.
I remember Falcon 3.0 and perhaps some other simulation games having support for FPU for those few who had one however...