Benchmarking slow LCP and TTI

I'm looking for advice on this, because I really don't have a mentor to turn to and I'm feeling pretty lost overall. I've detailed before that I'm refactoring a SPA made with CRA that is meant to replace an AngularJS app that was mounted inside a JSP and served as our previous "version" of this page. I've improved performance quite a bit compared to where it was, but I'm still seeing a rather large gap, and I'm trying to audit A) How much can I reasonably improve performance, and B) Where should I look for benefits at this point? For a bit more context - because the old application is in a JSP, it gets served HTML from our server which contains a pre-computed JSON string that is parsed and added to the page's service. The new version lives in an S3 bucket and sends multiple requests out for data upon page load. I originally thought this was the primary source of difference remaining between the two applications, because naturally sending to a remote server for data will be slower than parsing JSON that is passed alongside HTML. However, I experimented with removing those calls and simply parsing out a hard-coded JSON string with data, and I'm still reading a significant difference in mobile score (around 56 for the new vs 80-85 for the old page). When I profile the page (CPU 4x throttled, network fast 3G) I'm noticing a significant difference in the number of long tasks that appear to be falling under the umbrella of React-DOM vs the number that happen on the old AngularJS environment. I've attached screenshots of the trace, in case anything seems fishy. I genuinely don't really know how to interpret these results - does React-DOM often struggle when network/processing speeds are throttled, more so than this other app would? Any guidance would be invaluable to me. I'm feeling at a loss, and I don't know anyone that could help me with this, personally or professionally. I hope this question is acceptable, I'm sorry that it's abstract.