Thanks for the responses. "Glad" to know I'm not alone.
Regarding Craig's feedback, I use Waves and UAD plugins, both of which come with multiple instances of most, if not all, of their plugins. I also use a number of plugins that require an iLok. That said, those things were true prior to a month and a half ago when I started experiencing the extreme slowness, so, although I don't doubt that they contribute to slowing things down, it seems unlikely to me that they're the entirety of the explanation.
Also, I don't know that the 2018.11 update was the culprit. The fact that not everyone is experiencing it seems to suggest that there are other variables. I wish these things were easier to pinpoint.