[moritzwarhier]

Your statement is very broad. Might be the startup culture and lacking technological expertise, no?

Was insert-startup-product-built-on-.NET fast 15 years ago, for example?

Not trying to be controversial, just wondering.

I agree that a product built by clueless developers stringing together random libraries or bad custom code in React would create slow products.

In your spirit, I might argue that thinking "reactively" and client-centric tends to lead to unneeded requests.

Developing without meaningful API contracts or basic CS skills also makes it easy to unintendedly blow up network payloads.

Caching is hard though.

Have you worked on bad PHP applications that reload on every interaction without caching?

[zdragnar]

As someone who has had the misfortune of working on poorly tuned WordPress and Drupal websites in a former life I can confirm this is an issue of culture, not and axiom of whether SPAs are intrinsically faster or slower.