You don't. MP3 is not a DAW format, it is only used as a final delivery format.
Playing (yes, just playing) an MP3 actually costs more CPU than playing any noncompressed file such as AIFF, CAF or WAV.
The only MP3's that will not be converted by Logic are MP3's that were created (exported from, bounced from) by Logic itself. But even those you should convert if you're using them in a project.