AAC compression and playback take considerably more proccessing time than MP3. Also iTunes MP3 compression is
pretty crappy compared to LAME, Xing, FhG, and it's own AAC. I wouldn't use iTunes to rip to MP3, use AAC unless you don't care how it sounds. If you need to have MP3, use a ripper with LAME to encode, then import into your iTunes library.