Massive SUO file causes Visual Studio to hang when doing pretty much anything

I've been dealing with horribly slow performance from Visual Studio 2008 (SP1) for a few weeks now. We're talking seriously slow performance. Like 30-40 seconds of unresponsiveness to save the solution; 20-30 seconds of unresponsiveness after compilation if there were any errors.

Today, after taking a few memory dumps to send to Microsoft, I set out to look for network references (an idea from a workmate). I didn't find any (nor did I expect to), but I did find a 250MB (!) SUO (Solution User Options) file.

I have no idea how the SUO file got so large, but deleting completly fixed the problem. Visual Studio has returned to it's usual speediness. I do regret, though, not making a copy of the SUO to send to Microsoft. Next time, perhaps.