How much can misconfigured wordpress plugins stall your server’s performance

A couple of days ago I’ve met a guy who has a high traffic blog about tech stuff. The guy was telling me that he has hosting problems and that his blog is getting slower and slower by the day. I’ve offered to help him by providing hosting for him in one of the servers that I administer. After making the transition from his old hosting to my server, which was not an easy thing to do due to latin1 to utf8 conversions that had to be made – it deserves a post of it’s own, I started to notice increased load on my server. Sure his blog had heavy traffic…but could it be that bad ?