Mail.app slow/delay IMAP synchronization

Using IMAP with Mail.app is so slow. Even if I set it to check the email every minute, sometimes I getting emails every 15 minutes. Pressing "Get Mail" doesn't retrieve the emails as well, I have to right click on the mailbox and choose "Synchronize *email address*" to get the latest emails! Even my iPhone can get emails faster.

I am using Mac OS X 10.5.6