(Case 1052281) IMGUI.Controls.TreeView keyboard navigation is slow

The namespace UnityEditor.IMGUI.Controls contains a wonderful TreeView Control.

When navigating this TreeView with the arrow-keys, the TreeView becomes very slow, depending on the number of total items in the TreeView.

It is very fast and responsive if it contains only a few thousand items. However, if the TreeView contains one million and more items, it becomes unusable on my PC.

Unfortunately, several million items is an actual real-world-number in my use case. I develop a memory profiler that often contains A LOT of items in large projects:
https://forum.unity.com/threads/wip...filer-debugger-and-analyzer-for-unity.527949/

Spoiler: Profiler Screenshot

Reproduce

• Open user attached project
• Open Mainmenu > BugReport > Open TestCode window (this takes about 10 seconds on my PC)
• Open Mainmenu > Window > Debug > Profiler
• Turn on "Profile Editor" and "Record" buttons in Profiler window
• Select the first TreeView row in the TestCode window
• Hold down "arrow-down" key

Observe how unresponsive the selected item changes in the TreeView. Observe in the Profiler that "MyTreeView.OnGUI" often causes up to 1 second spikes.


Expected
The TreeView should not get slower with increasing number of items.

Actual
The more items in the TreeView, the slower the keyboard navigation becomes.