You certainly can build slow apps with React, it doesn't make building slow things that hard. But honestly, React primitives (component mounting/unmounting, rendering, virtual DOM diffing, etc.) just aren't that slow/inefficient and using React in a fairly naive way isn't half-bad for data-heavy apps.

I actually have been trying to figure out how to get my React application (unreleased) to perform less laggy in Safari than it does in Firefox/Chrome, and it seems like it is related to all the damn DOM elements. This sucks. Virtualizing viewports adds loads of complexity and breaks some built-in browser features, so I generally prefer not to do it. But, at least in my case, Safari seems to struggle with doing certain layout operations with a shit load of elements more than Chrome and Firefox do.