The clips are all pretty long. 1 machine has 32GB and the other has 16GB of RAM, so the load is not nearing maximum. The MacBook Pro is a 2018, and the iMac isn't far behind, so the specs are up to date.
Here's the irony... when I load all the clips from disk instead (turn off the RAM option on clips), the problem goes away completely.
I probably don't need to have the clips loaded into RAM, as I've got SSD/flash storage on both machines, but this seems odd to me. I'm not sure why RAM would cause the project to grind to a halt. Is Ableton just not meant to hold that much audio in RAM? Is this RAM feature for clips only for small files? Whatever the explanation, if you're going to have a RAM option for clips, it should work no matter how you use it as long as you're not running out of RAM. Am I crazy for thinking this?
My hope is that this is a bug of some kind, and perhaps someone else can verify that this happens to them also. We've got all this RAM these days, so why not use it right?