http://www.getthunderbird.com/

Thunderbird is a free, extensible mail client software with excellent IMAP support.

https://support.mayfirst.org/wiki/speedup_thunderbird_imap

http://luxsci.com/blog/optimizing-mozilla-thunderbird.html

See Thunderbird Synchronization

http://enhancedlinux.com/2010/01/15/disable-thunderbird-3-bloat/

Thunderbird caches local copies of your message store in your Windows profile. If you have Roaming Profiles, this can negatively affect your login/logout times.

However, the local caching is a significant speed enhancer.

Edit (Tools) → Account Settings → Synchronization and Storage

Delete cached files taking up space:

C:\Documents and Settings\<profile>\Application Data\Thunderbird\Profiles\<profile>\ImapMail\*