I mean that it's not usually the language or the framework, but what overall interactions look like. If static HTML loads slowly, yes, it's the backend, but that's not what most sites look like these days; they're rich, client-side apps making lots of API and resource requests, not to mention overhead something like React adds.

It’s extremely app-dependent. For example, lots of heavy analytics, data crunching type apps can easily have multi-second (or longer) backend requests. For these, the BE tends to dominate perf issues.

However, for your standard CRUD type app, honestly round trips of 100-200 ms for backend requests are really common/standard. On the FE, loading and evaluating big JS bundles, rendering, etc., can be way slower. For CRUD apps I’ve worked on, perf issues on the FE have been more common than on the BE. Especially with the common React/Redux stack, it’s very easy to make perforce mistakes and end up with a lot of unnecessary component updates when things aren’t really changing.