Although it is stated on this page "You can also use them to simulate statistics in the Editor, for example, an in-Editor health bar", there is a notable overhead to using EditorGUI.DrawRect which ...

Unity 5.4 以前の仕様なので、根本的な対処は不可能。5.5 で自然に直る。 ただ GameCanvas 側で回避手段を用意できなくもないので対処を検討。