Right click the account and go to Synchronization and Storage. Double check on individual folders. I know this works because my ImapMail folder (in the profile folder) is much bigger than it would be if it only had subjects, and a quick sampling shows full messages.
If you have this on and it's still taking seconds, the problem is probably coming from something else.
If you have this on and it's still taking seconds, the problem is probably coming from something else.