This is a straw man. There's no such thing as a "purely server rendered app", and never has been (except maybe in the pre-JS days of the internet). At the end of the day, all webpages are produced by a server, and rendered by a client.
The only reason that server-rendered apps are "slower" is because people don't think about what they're doing, and leap right to 100% client-side rendering. Very few things actually need the latency guarantees of client-side rendering.
The only reason that server-rendered apps are "slower" is because people don't think about what they're doing, and leap right to 100% client-side rendering. Very few things actually need the latency guarantees of client-side rendering.