The webpage www.ffmpegx.com/benchmarks.html lists some (old) benchmarks for the encoding speed.

The encoding engines have changed a bit since v0.0.9s, but perhaps not a lot. I think my tests say more about the progress that Apple/Intel has made with newer chipsets. I believe there is room for improvement with updated binaries that may be able to take advantage of the design of later Intel chips.
I do not know which movie was used for the older benchmarks, so I hope my test movie isn't very different with regards to decoding/encoding speed. I did use the full movie, to rule out selecting a segment with characteristics that could be influential.

iMac i7 3.4 GHz
mencoder mpeg-4 [AVI] (crop 640x272): 372 fps
mencoder mpeg-4 [AVI] (no crop): 349 fps
mencoder mpeg-4 [AVI] (crop, hq): 324 fps
ffmpeg mpeg-4 [AVI]: 167 fps
ffmpeg mpeg-4 [MOV]: 168 fps
mencoder (hq, trellis, b-fr., bilin., SSE): 153 fps
ffmpeg XviD [AVI]: 143 fps
mencoder XviD [AVI]: 153 fps

The overall results are the same as the old benchmarks: mencoder can be faster than ffmpeg for DVD to DivX/XviD, although selecting many options does slow it down considerably. The slowest on my test was 00:21:26 encoding time for this 02:02:13 movie. I must admit to have a few other apps open at the time (Safari (no java, no video), TextEdit and Calculator). And ffmpegX Progress wasn't always the front-most application (not sure how much difference that makes).