One similarly ignored bug that springs to mind is the performance of the "Send To" context menu item in File Explorer. I always dreaded dragging my mouse over it by accident.
Apparently nobody at Microsoft thought about the Windows registry as a database, which of course needs indexing to be performant.
They could also cache that computed menu and proactively update the cache whenever the relevant keys are changed. Either way, pretty far from "cannot be fixed without breaking the API".