Though this is primary about an electron app, there's some very useful insights in this article on performance and changes that have direct impact on browser based load times - a useful read even if you're dipping in and out.

How to make a React app load ~30% faster – by tuning some configs and delaying some scripts

Single biggest win: if you're not code splitting, you're hurting your customers, which in turn, hurts your own business.

Source: 3perf.com