Squirrelmail is a bit pants unless you do the following:
- Use a good IMAP server. I like Dovecot!
- Increase PHP's memory limit from 8 to 16M. After doing this, I found that I was able to load folders with large numbers of messages.
- Turn on support for server-side sorting. Dovecot is much faster at sorting.
- Use RoundCube instead. It's far nicer!