Collider 2D in the shape of arch.

Hi
I'm trying to build a simple 2d game for android and now I'm stuck.
I have a shapes looks like circle with gaps (pic bellow) and I need to add colliders basic on circle but with gaps.

Important information is, in game that objects are rotating all time.

I tried to do it using polygon collider and in theory works good but only if there was max 3 objects with this colliders. If there is more then 3 then on Android game had max 13FPS (more objects less FPS).
So polygon collider doesn't work due performance reason.

Probably to have a better performance I need a collider in the shape of arch, but how to do it?

 

A little late after 2 years, but I found this post while searching a similar thing, so as this was one of the top 2 results, a guy on the Brackey's forum coded a 2d collider that does just this :

http://forum.brackeys.com/thread/unity-custom-2d-colliders-scripts/