Large quantity of misaligned stories cause Program Room interaction to slow down/hang

Issue Summary

Large quantity of misaligned stories (~700+) cause Program Room interaction to slow down/hang.

Steps to Reproduce

Misaligned Stories: Stories that are assigned to a sprint within the PI being viewed, while their parent feature is assigned to a different PI.

1. Create a Feature, assign it to a PI
2. Create ~1000 children stories in the above Feature, assign them to a sprint in the above PI
3. Change the PI in the Feature to unassigned backlog
4. Go to Program Room and select the above Program+PI in tier1 filter
5. Wait for Features and Stories to load in the report below - should see 1000 misaligned stories
6. Open/close side menu a few times - observe it is really slow
7. Same for any other interactions on the page that do any sort of data fetching, like opening a work item or even tier1 filter
8. Align the Feature/Stories again and the problem goes away

Expected Results

Report and UI should not be slow.

For example, if we have 1000 Orphaned Stories instead of the misaligned ones, then the report is completely fine.

Orphaned Stories also don't load all at once, 'Show More' button loads them in batches.

Actual Results

UI becomes gradually slower and hangs longer after ~500-700 misaligned stories, although it's not very noticeable until 400-500 misaligned stories.

Misaligned stories are all loaded to the page at once, there's no 'Show More' button like for Orphaned Stories.

Workaround

Align as many misaligned stories as possible by making sure the parent Feature has the same PI as the stories' Sprints.