@maxz I explicitly set up both the current behavior and the previous behavior just for you. Really. Thumbnails do eventually get updated, just not immediately. I limit how often thumbnails are updated to improve overall performance.
Remember this thread? Layouts slow down drafting
In the modeling window there typically are four viewports that get updated with every change. In the layout window, you can often have 3 or four viewports per page, so this doubles the number of viewports that are updated with every change.
For you, going from updating 4 modeling viewports to updating 8 viewports (4 modeling, 4 layout) slowed things down. Back then, no layout thumbnail was updated immediately.
Consider that every thumbnail is another three or four viewports, so constantly updating all the thumbnails will require updating 10 to 20 to 30 viewports, depending on the number of layout pages (and thus thumbnails) you have. Updating all the thumbnails all the time would likely make modeling on your older Mac pretty frustrating.