I think it's just the way web apps are architectured. If you use vanilla JS you can make the browser render stuff really fast even if you change the DOM, as long as you do it efficiently (don't update ALL the DOM on every change!). But I bet Google uses some reactive framework that keeps a bloody copy of the whole DOM in memory, so when you throw 30MB of content into the DOM, that copy that was meant to make things faster starts being the bottleneck.