So some more details...
As mentioned disabling Jetpack seems to make the problem less severe (it's the only plugin that has a marked effect on performance) but it certainly doesn't fix it.
I've tried absolutely everything I can think of and not only that but every piece of advice I could get from hosting providers, friends, forums, etc. No dice. I have tried the site now on four different servers so I think it is something in the WordPress code but this seems ridiculous because the only changes I've made are: 1) Add articles, 2) Plugin updates as they become available, 3) WordPress core updates as they become available. The problems appeared on or about Dec 20.
More details to come.
