While using kmail, sometimes a lot of disk activity starts to happen. From top, I can tell that it is kontact that is causing the disk activity (since the process is marked as disk wait). While this is happening, kontact becomes unusable, and the rest of the computer is very slow as well.

I'm using a dimap account for a google email address. My computer has enough ram, so it isn't due to kontact hitting virtual memory.

It seems to mostly occur when opening a message from a folder which has a lot of messages in it. It also sometimes happens on program startup and checking new mail.

On a possibly related note, some of my messages are losing their date info (it is replaced with unknown). Also, I have a few messages with no content, but (unknown) as the subject and sender.

I'm running kde 4.2.4 with kontact 1.4.4, under archlinux

This is because Kontact ( KMail actually ) is downloading and caching the contents of those folders on disk. I believe it creates one file per email.

With regards to the mysterious messages with no subject/content/unknown dates it is likely a corrupted cache, which is likely part of the reason why it is having issues.

Please remove the GMail DIMAP account and recreate it.

Thanks for the tips. I should have mentioned in the question that I have been using this dimap account for a few months now, and it hasn't been slow before (it was slow for the first sync obviously but fast otherwise). Do you think this is relevant?

I'm a bit reluctant to recreate my dimap account, since it will require hundreds of megabytes to redownload all the messages, and I have a fairly limited download cap. I have been keeping incremental daily backups of all my files for about 6 months, so I can revert my kmail and kmailrc files to a time in the past. Is there any way to check if any corruption has appeared (I could use this info to revert to the latest "good" snapshot)?

I would try reverting to the day or so before you started encountering this issue.
You need to revert the directory $(kde4-config --localprefix)/share/apps/kmail as well as the configuration files.