This reminds me of a situation I've seen and found myself in far too many times. When I encounter a performance problem I immediately think about areas where, during initial development, I stopped and said "this might cause a problem with a large amount of data" versus doing the proper thing of actually hooking up a profiler or doing proper testing to determine what the actual problem is; Rarely is it in an area I previously identified as potentially causing future problems.

Ego (I know what the problem is!) and the drive to fix problems quickly always get in the of finding the solution. Tales like this are a great reminder to not assume you know what the problem is.