It's even more for me, it looks to me like the embedded React sandboxes are essentially full React apps in development mode. So none of the usual optimizations for size are active, and you get the full devel-mode bundle for several React applications on this page.
When I checked it was 46MB! Looking at the network tab, it seems to be making a bunch of extra requests for the same assets.
I know that Vercel/Next.js will lazily load in certain JS chunks but there seems to be some mechanism that is loading a ton of stuff up front. Edit: from other comments in this thread, it's the sandbox code, really crazy how much overhead that adds to the page!