Steps to reproduce:

1. Create new project
2. Open asset store window and make sure it's docked
3. Try resizing it and notice that it's laggy

Reproduced with: 5.6.0p1, 5.6.0p3
Not reproducible: 5.5.3p2, 5.6.0f3, 2017.1.0b2

Additionally, undocking asset store throws:
InvalidOperationException: Operation is not valid due to the current state of the object
System.Collections.Stack.Pop () (at /Users/builduser/buildslave/mono/build/mcs/class/corlib/System.Collections/Stack.cs:329)
UnityEngine.GUILayoutUtility.EndLayoutGroup () (at /Users/builduser/buildslave/unity/build/Runtime/IMGUI/Managed/GUILayoutUtility.cs:323)
UnityEditor.HostView.EndOffsetArea () (at /Users/builduser/buildslave/unity/build/Editor/Mono/HostView.cs:190)
UnityEditor.HostView.InvokeOnGUI (Rect onGUIPosition) (at /Users/builduser/buildslave/unity/build/Editor/Mono/HostView.cs:238)
UnityEditor.DockArea.OnGUI () (at /Users/builduser/buildslave/unity/build/Editor/Mono/GUI/DockArea.cs:346)