Quote - The only thing I can think of is the full text indexing engine. Maybe it causes the site to slow down when it's updating the catalogs.

That definitely seems worth looking into. I develop with SQL Server, and I'm not terribly familiar with MySql. I'll do some digging into it, and see if that's a common problem elsewhere. I don't know how scalable MySql is, either... there is an awful lot of data here to index in the first place, I'm sure.

I wonder if the full text engine runs in a separate process, or if it can be made to do so? If it's causing a problem, perhaps its process priority could be adjusted to mitigate the effect.