This is not why half the internet is borderline unusable. React will happily do a full page render in under a second (including network load time). The primary reasons for that are:
1. People writing code that does many network requests in series. 5-10 seems to be not uncommon for many apps.
2. Adverts and as SDKs.
The app I inherited at my last job was a huge 2MB bundle (due to including both the firebase SDK and a large charting library), which I assumed was the cause of the slowness (it was taking ~3-4 seconds to load). But it turned out that was serial network requests that were slowing it down. The app is still 2MB (we made good progress on removing firebase, but hadn't quite gotten there when I left) but it now loads in around a second.
React is a small library that loads quickly for most people.
Like 30% of pages just give me something along the lines of "enable js to make this site work"