https://www.quirksmode.org/dom/innerhtml.html

https://segdeha.com/experiments/innerhtml/

The thing which was slow is changing more than you need to change. When the React marketing push included performance, that’s almost always what they were comparing it to - someone would have slow code making the same updates multiple times or triggering reflows by forcing the browser to do a partial update to measure something, and then updating again.

There’s an argument that the guaranteed overhead of using React is better because it helps the average developer who doesn’t measure performance much do a better job than they might have otherwise, and that can be true for many people but it also tended to get oversimplified to “React is fast” despite widespread evidence to the contrary.