One thing which surprised me at a recent job was that even what I consider to be a large bundle size (2MB) didn't have much of an effect on page load time. I was going to look into bundle splitting (because that included things like a charting library that was only used in a small subsection of the app). But in the end I didn't bother because I got page loads fast (~600ms) without it.
What did make a huge different was cutting down the number of HTTP requests that the app made on load (and making sure that they weren't serialised). Our app was originally going auth by communicating with Firebase Auth directly from the client, and that was terrible for performance because that request was quite slow (most of second!) and blocked everything else. I created an all-in-one auth endpoint that would check the user's auth and send back initial user and app configuration data in one ~50ms request and suddenly the app was fast.