Well, I spent a couple hours hunting down the issue. I rebooted in safe mode and the problem was gone, so it had to be something that could be cured. Ran diagnostic, rebooted without anything plugged in the back. Lagging was still there. Then a thought occurred, maybe it's a problem with the wireless keyboard, not the Mac. I simply turned the wireless keyboard off, then turned it back on. I haven't seen the lag since. 🙂