Serious question: what is the issue with these paritcular sizes? I know that features/look these websites have are definitely achievable with less JS at a higher engineering cost, but what's the problem with it? 10MB loads in two seconds on an okay-ish desktop connection (correct me if I'm wrong, but most of people don't deploy Vercel apps from their phone from a mountain range with 3G connection). The experience on the websites mentioned is smooth as it can only get; everything is super fast and nice. Every subsequent click is just instant action. That's how web should look like.

Is the problem here that they perform poorly on slower computers/connections? Is it even true? Is there an audience of developers who can't use Vercel or GitLab productively because of that? Any metrics to support that? IMHO optimizing against bundle size/JS sent over the network is one of the worst metrics for performance I could imagine.