You also notice that it is doing a lot more reads than it should, and by a lot, I mean hundreds of millions versus tens of thousands. Now stop right there…
Without knowing anything else, what’s the first thing that comes to mind? Don’t worry. I’ll wait.
Statistics! Yes, stale statistics. “But, Stacy,” you protest, “we have a job that runs every night that checks the fragmentation of the indexes and rebuilds the fragmented ones. This cannot be. You must be terribly mistaken.”
Check them anyway. In this case, it was, indeed, the statistics. Update stats with full scan resolved the issue right away. Remember, when things were running fine a couple of days ago, and now, for seemingly no reason, things are running slow, check your stats.