> Only if a dev has screwed up. Most simple sites are fine. You have to push the DOM quite hard for the browser to be the bottleneck. I've worked on apps that have DOM trees with 60,000+ nodes that remain under 16ms (because very few were actually changing at any given time..)

You mentioned that frameworks like React make it simpler to get better performance but based on my observations it's quite the opposite and React is often a double barreled gun where one of the barrels is constantly pointing at your feet. With vanilla JS (not the one from this post) you have to put effort to get everything to update properly. With React you put effort to get the least amount of things to update. It's not even about DOM size but about managing state and how much code gets run on state change.

To give you an example - Reddit is a simple website. You have a list of posts which are either text, image, or a video and each of them has comments. Absolutely nothing complicated about that, yet the performance on their React frontend, even with all analytics JS and ads blocked, is horrendous. You would assume that at Reddit's scale they'd be able to hire competent React developers.