Steps to reproduce:
1) Open user attached project
2) Open profiler -> enable editor profiling -> start recording
3) Open Window > Texture Overview
4) Hold down middle mouse button in Texture Overview window and move mouse to pan the view(panning in circles outlines the issue best)
5) Stop recording and observe profiler

Expected: performance is as smooth as in 5.3
Actual: it is choppy and the draw calls are a lot higher (was able to get up to 256)

Unity 5.3 > EditorWindow2.OnGUI > 4 Calls
Unity 5.4 > EditorWindow2.OnGUI > 28 Calls