Do you know what kind of server you're trying to connect to? Not all of them support push-to-client, which is what the iPhone uses by default to appear as though it checks automatically. (It seems unlikely, but if you did a software update or had to go back to factory settings and restore your data from a backup on your computer, it may have bumped you over to the manual setting, or turned on push when your server doesn't support it.)
If you set it to check periodically, you'll get less battery life. It's in Settings, under Mail, Contacts, Calendars > Fetch New Data. (It's right underneath the list of mail/directory/calendar servers.) If you have multiple email addresses you use it with, at the bottom of the fetching preferences, there's an Advanced option that lets you specify settings for each individual account.
Mine acts funny with the index page - I think it's the chatbox that slows it down. Going straight to the subforums, the New Posts page, or closing the embedded chatbox resolves all of my problems.
~Joe
