BetterZip uses the unrar library by Alexander Roshal, creator of RAR. I have also observed that the extraction speed greatly varies from archive to archive. I have no explanation at the moment, but I will research this further. Of course, Windows is the native platform for RAR, so maybe its author optimized it for that system. Have you tried other unrar utilities on the Mac, like UnRAR X?

I have observed that BetterZip is a fraction of the speed on the commandline unrar when extracting large files (DVD ISO multivolume). I have not tried to compare small files as its too fast to easily notice. What takes unrar 2-3 minutes to extract takes at least 10 minutes with BetterZip. This is on an original 2GHz dual G5 with 4GB RAM running 10.5.5. If the decompression routines are straight from the open library, then perhaps the difference lies somewhere in file I/O routines (different read/write sizes, buffer size, etc).

Yes, BetterZip uses the rar library which also handles file I/O. I'll look into this anyway.