I think I've seen this before - both when my PC had been very low on memory, and with files with lots (like, thousands or tensof thousands) of objects on a page... especially when they've been imported from an EPS or PDF file. From memory, having selected everything and grouped it all has helped.

What about copying everything from your old document and pasting it into a new one, does that help?

I don't remember it happening recently... like, not since the pandemic. I don't think the type of work I do has changed, but thinking about what might have, I do use the Pro+ software so perhaps it's a bug that has been fixed. I do now have a very fast desktop PC, so perhaps pure processor grunt can overpower the "Processing..." if it still happens.