Hard to answer that with concrete numbers but Om Next handles optimistic and non-optimistic updates quite elegantly. Roughly speaking: each query expression is parsed twice, once locally and once in remote mode, and as long as your backend can handle Om Next query expressions, it Just Works:

Reading: https://github.com/swannodette/om-next-demo/blob/master/todo...

Mutating: https://github.com/swannodette/om-next-demo/blob/master/todo...

Depends entirely on your queries/mutations and app requirements. Om does produce regular React components, and due to immutable by default collections and data, Om can even beat React when it comes to rendering performance. At the very least, you should expect React-like performance. Then again, I've written a couple of frontend applications, and React/Reagent/Om has never been a problem for me from a performance standpoint. If it was, ClojureScript makes it easy to write code that uses regular js-objects and arrays, so it's always possible to optimize with "low-level" code when necessary. Like I said though, it has never been necessary.