Something else to check is to see what's in your Log Viewer.  For two reasons:  if you have 2 million rows in the DNN site log, that would severely bog down your performance, and there may be something that helps you trace your problem.

I actually had the same thought.  But, as I said, the reason was two-fold:  to see how big it was and to see if there were any helpful logs.

For example, on my end, my database went from ~50 MB to over 600 MB in a short period of time (not overnight, but in the course fo about 5 - 8 days) and the site was slow (one or two seconds to begin loading a page).  I looked at the log and found that it was logging an error for every page load.  Fixed the error, cleared the log, and everything started working fine.

Since then I've also implemented a database maintenance plan that seems to be helping, too.  I don't clear the log on a regular basis, but I have SQL server run a db cleanup & compact once a week.  You can also schedule a SQL job using DNN's scheduler to, say, clean out logs older than two weeks - I believe Mitch Sellers had a post about this.