I&#x27;ve had a couple users complain that my program started running really slowly while they were using Microsoft Security Essentials. It turns out that if a file contains a &quot;suspicious&quot; pattern of bytes (which in these two cases were two different user-drawn bitmaps), SE will do some time consuming heuristics every time you open the file. So opening, reading, closing a file 10 times in a row is really bad.