..., slow and resource-hungry, prone to crash, ...

Slower than pages with HTML+CSS with tiny bit of JS. Much faster and leaner than current JS SPAs.

This is the right way to look at it. Anything with a ton of JS ads chewing up memory now is just as bad, or worse, than a ton of Flash ads were in the early 2000s. But Flash sites were, essentially, SPAs at the time. And they were far more performant than most JS SPAs are now (although a lot of that also has to do with React/Vue/etc. and how poorly-trained coders choose to deploy them).

IIRC Firefox was the first one to move extensions to a separate process so a hung/crashed extension wouldn't take down the whole browser.

That's when I learned about the halting problem.