A quick update.
I have found that if in the ViewController with my ARView I override viewWillDisappear(_ animated: Bool) and remove the anchors added to the scene before the scene disappears, then the meshes do drop back down to normal when I reload the ViewController
where worldAnchor is the root anchor for most of my content.
HOWEVER, eventually FPS does drop even though the mesh count is low again. That is, the more times I reload the ViewController with the ARView, the slower the FPS becomes.
So I still don't know how to correct for that.
I took a cue from this old thread
and built a very simplified UIViewController with an ARView and repeatedly went back and forth loading and unloading the view and displaying my somewhat complex model.
Everything worked fine, so the problem in my larger program is clearly mine. I suspect my code is complex enough that I have one or more accidental retain cycles going on.
I am going to close this thread.