You are correct that loading times are not valuable for B2B apps or apps behind a login. But the main issue is that there is no simple or straightforward proces

You are correct that loading times are not valuable for B2B apps or apps behind a login. But the main issue is that there is no simple or straightforward process to achieve performance in load time / run time if its not followed from the start. You build the same capabilities but they are just smaller. I worked for an analytics product that never really bothered about performance on load or any of the web vital metrics. When they wanted to strike a deal with a fortune 20 customer, they were questioned on the performance of the app. It was not good. Took around 5 to 10 seconds to load everything on a macbook pro. Now all of a sudden we needed to fix the performance for all the pages. I helped set up the processes we needed to follow to measure and focus. This was one of the most complex scenarios you can face in an Enterprise product. Touching old code for the sake of speed. btw, the biggest perf improvement we realized was when we switched from React to Preact. There were some issues to iron out but it was okay considering the perf benefit.