I can’t tell exactly what the root cause of this is, but when I choose the pathfinding gameObject in my scene and view it in the inspector, Unity’s CPU usage creeps to 100% and Unity starts lagging until it becomes unusable. As soon as I de-select the pathfinding object it’s all fine again.
Ah, it’s already been noted before: http://arongranberg.com/vanillaforums/discussion/1193/startup-cache-dramatically-slow-down-unity-editor