Not in the slightest. Speaking categorically, JSON + templates are smaller to transfer over the wire than fully-rendered HTML -- and can be cached in exactly the same way ... even bootstrapped into a single HTTP request.
If you care about optimizing it, you can get your JS UI to render faster than the equivalent pre-rendered HTML would have.
If you care about optimizing it, you can get your JS UI to render faster than the equivalent pre-rendered HTML would have.