I've got an AVI file with an MPEG4 video, encoded by XviD.

An HE-AAC .AAC file encoded with foobar/nero.

I tried mp4creator60 as follows:

mp4creator -create=movie.avi -rate 25 temp.mp4
mp4creator -create=audio.aac -rate 25 -I temp.mp4

But the resulting file seeks extremely slow. I've tried adding -H, but that added significant overhead and didn't fix the seek problem. Help?

Mp4creator60 can’t correctly handle XviD 1 with b-frames. The resulting mp4 file it seems to "virtually" have only one keyframe, the first one, so probably a player has to decode it from the beginning.

IMHO the best way to mux XviD 1 into mp4 container is 3ivX muxer.

Be aware that 3ivx has some problems decoding XviD’s b-frames. It should be fixed in the next release, but the muxing procedure should work fine.

Personally, I use Nero ShowTime decoder filters and CoreAAC for decoding such material (xvid 1 in mp4 container) with Media player classic

Hope I helped a little.
Forgive my bad English.

Ok, first of all I would not use packed bitstream in XviD if you target file is mp4. 3ivx muxer can remove these but it is best just to not use them, they may be the source of your problem. Also, XviD seems to be having problems with mp4 and N-VOPs (dropped frames). So if you are using cartoon mode or the frame drop and b-frames together this will produce very weird problems most likely desync but this doesn't sound like your problem but I thought I would bring it up anyhow since it is worth mentioning. In RC4 you cannot use dropped frames and b-frames together now because of this. If you are careful and mux XviD files without packed bitstream and n-vops your files should be fine in 3ivx demuxer, nero, etc. Keep in mind that you do not need to reencode those files, just mux in the 3ivx muxer in graphedit.

Can you give a brief description how to use the 3vix muxer?

First, get the D4.5 pack from http://3ivx.com

The get Graphedit from Doom9 or Digital-Digest.com, download section. Run Graphedit and goto 'file' 'render media file' and open your source AVI. Then delete all filters after the AVI splitter/parser filter, goto 'graph' 'insert filters' and insert the 3ivX muxer filter ( should be in 'DirectShow filter' ) and a file writer filter. Press the play button ... wait .....

http://3ivx.com/support/windows/encoding/ge_avi2mp4.html