I recently found a scalability problem in here, where if the banlist is large (for example, the forum is using a ready-made list of known forum spammer IPs) the function would make an ambiguous query, which could include many thousands of result rows. This would cause slow execution time, and possible PHP memory limit exceptions and/or maximum execution time exceptions, not to mention high memory and processor usage in general.

I have written a patch for the issue, but don't know what the right channel to go through to submit it is on phpBB.