Adding a link to my investigation of this bug. There a lot of synchronous operations we are doing on the main thread in multiple places during the import process. We need to solve those as well as optimize the rendering pipeline for login items.
I don't believe solving these performance issues will result in particularly novel solutions. Because of this, we should have some resources on file on how to mitigate and solve these kind of performance issues. If we don't have these resources, then this bug could be a good case study of how to use the profiler, moving main thread IO off the main thread (or minimizing main thread IO if we can't move all the work off the main thread), and optimizing the pixels to screen pipeline. The performance best practices for Firefox front-end engineers highlights these issues at a high level so maybe this bug can expand on those high level points.