Its not really a trope, the opposite is much more common. People are much more quick to mindlessly blame SSR for slowness like with ROR or PHP.
The reality is both can be slow, it depends on your data access patterns, network usage, and architecture.
But the other reality is that SPAs and REST APIs just usually have less optimal network usage and much worse data access patterns than traditional DB connected SSR monoliths. Same goes for micro service.
Like, you could design a highly scalable and optimal SPA. Who's doing it? Almost nobody.
No, instead they're making basically one endpoint per DB table, recreating SQL queries in client side memory, duplicating complex business logic on the front and back end, and sending 50 requests to load an dashboard.
React also has a class of problems that doesn't really happen in other types of apps: re-renders.
Even other frameworks like Vue.js, Solid or Svelte don't really suffer from it as much. It simply happens a couple order of magnitudes more often in React than any other framework.
The reality is both can be slow, it depends on your data access patterns, network usage, and architecture.
But the other reality is that SPAs and REST APIs just usually have less optimal network usage and much worse data access patterns than traditional DB connected SSR monoliths. Same goes for micro service.
Like, you could design a highly scalable and optimal SPA. Who's doing it? Almost nobody.
No, instead they're making basically one endpoint per DB table, recreating SQL queries in client side memory, duplicating complex business logic on the front and back end, and sending 50 requests to load an dashboard.