Thunderbird 32bit works perfectly well on 64bit system. Slowness can depend upon a variety of possibilities. Background processes eg: updates for OS, Adobe. Java etc Anti-Virus products will interrupt and scan causing some lag. Suggest you do not scan Thunderbird folders on startup nor bother scanning incoming or outgoing mail. Thunderbird does not show remote content by default. Only open attachments you are certain about and AV should scan those on opening anyway. IMAP can be slower than POP for obvious reasons as there is much synchronising going on. Keeping your thunderbird emails organised into smaller folders is better than a larger folder and maintained by compacting. Useful links to read: * http://kb.mozillazine.org/Performance_-_Thunderbird * http://kb.mozillazine.org/Keep_it_working_%28Thunderbird%29 * http://kb.mozillazine.org/Compacting_folders