You might have switched on McAfee's scanning of emails, which with OE is a complete disaster in performance terms.
My message base is only a few gigabytes, but with McAfee scanning turned on it can take tens of minutes for OE to start up. Turn off the McAfee scanning and it goes back to normal.
This behaviour of McAfee is completely pointless of course - there's no reason to scan the tens of thousands of historical messages in the message base that have already been scanned countless times, the need is just to scan incoming messages. I haven't found setting that does that, so I just leave McAfee email scanning turned completely off.