Its also one of the most immediately visible degradations of user experience. The cursor is your agent to interact with the computer, an extension of your arm basically. It should be one of the most smooth and responsive pieces of an OS.

I remember a long time ago, early days of Gnome 3 when doing large file operations would hang the cursor. It made the whole system feel like complete garbage, totally unacceptable.

A cursor lagging (or any kind of GUI/user interaction stutters) should be a critical high priority bug, all hands on deck sort of thing. The entire experience of using the damn thing hinges on the responsiveness of the interface and pointing devices.