Importing m4a files (AAC or ALAC) should not be considerably slower than importing other audio formats, but m4a tagging is often slower than tagging some other formats like MP3, FLAC and APE. As far as I know, MC always needs to rewrite the entire files when it applies m4a tag changes. MC can tag some other formats faster if the tag change doesn't require rewriting the entire file.
To troubleshoot your problem:
- check that a security program is not unnecessarily inspecting each accessed m4a file.
- if you are importing through a LAN connection (remote computer, NAS, etc), try importing from a local drive (as a test).
- disable file analysis and cover art lookup in the import options (could it be that your MP3 files have already been analyzed for Replay Gain?)
- disable tag writing globally (to test if this makes any difference): Tools > Options > General > Importing & Tagging > Update tags when file info changes (uncheck).