NYHorsewoman is right. It depends on the email server/service you're using. For some, a really big "trash" or "junk" folder (or other frequently accessed folder) may be the cause of the bog-down or lock-up. That's why some email services and clients "archive" or compress old mail sitting in the folder forever.
Try clearing the "junk" and "trash" folders, as they often tend to be the ones that fill up the most and the fastest. Good email housekeeping is a plus. Spend a little time going through the one other folder that loads up easily: the INBOX. Get rid of the cruft and file away stuff you really don't need to keep there long-term. For exchange and IMAP servers, etc., the inbox folder is the one synced most often.
Michael "Spam, spam, bacon, eggs and spam. Hold the bacon and eggs." Sent from my iPad 2 using iPF