There is ample evidence to suggest that Cloth physics had a massive performance-decreasing bug in them in Unity 5.6 and Unity 2017: https://forum.unity.com/threads/case-977420the-performance-of-cloth-physics-drop-dramatically-since-unity-5-6.508398/
Now that 2018 is being updated to, please consider tweaking the performance rank values for Cloth verts. The current values are based on the 5.6/2017 "baseline", which is a lot heavier than in 2018.