I've read that the C2P takes up less than 5% of CPU time in Amiga Doom. There are other reasons for the Amiga version being slow.
When I had an A4000 I had the 68k version running on my 060-50 about 23FPS in AGA, around the same as my old 486 SX PC.
On the PPC (150Mhz 604e) and mediator + Voodoo 3 I reached 46FPS, about the same as a P90.
