The slowness described in the article isn't jank, but time-to-initial-load. Conceivably the initial client-side render could take a nontrivial amount of time, b

The slowness described in the article isn't jank, but time-to-initial-load. Conceivably the initial client-side render could take a nontrivial amount of time, but in the vast majority of cases - just like for server-side rendering - it isn't a deciding factor. The real deciding factor is the amount of dependencies that have to be loaded. This includes JS, but usually the JS for ads/tracking is far larger than the JS for the actual UI, even on fully client-side-rendered sites.