Using the flipping code above, my port of Doom at 304x224 (TV mode) gets between 50 and 70 FPS while running the timedemo, spending most of the time right about 60 FPS. When playing the game, it stays a rock-solid 35 FPS the whole time (the code I work from has the frame limit except when running the timedemo). So the multi-buffer screen flipping is only as slow as your app is at drawing frames.
In case you're wondering, I have played it at 608x448 (TV mode again), and it gets between 15 and 25 FPS at that resolution. That's a TRUE resolution - Doom is actually drawing at 608x448, not a lower res and then scaling up. The diagonals are noticeably cleaner.
If you want to try it, here's an alpha. Everything SHOULD work, and mostly does, but weird things happen sometimes, all of which have so far been compiler problems.
DCDoom-cw-alpha1.sbi
That has the shareware wad file. If you just make the disc image, you can just immediately start playing. There's a nice readme in the readme directory. Source is included, as always.
