You gave a link to an image. By capx i meant the file .capx of your project or a more simple one so I can make some changes, maybe find a solution, and give it back to you, so you can compare.
For now, I don't know what's the cause of the slowdown. In construct 2, slowdowns are commonly due to objects having too much behaviors at the same time that check for collisions. So for hundreds of objects you can have thousands of collisions checks per seconds.