It is not that it is too many, the collision on them is not needed. what are there 200 trees there? that's 200 less things for the engine to do right off the bat, and the player will know no difference.
In stead of going into each one and changing the physics to off, maybe delete all those trees, (still a process) and then set the first tree again, set its properties so that physics is off, then extract it and then reset or spray them. The Physics off alone should be a significant savings. Plus the LOD of the trees should make it a lot less polys. So I am still betting on the physics over head of those to start.
I would still walk around the map with the first Tab matrices on and see where the build up is in poly count and if it has anything to do with the slow down.
You can also slide the debug slider above 10 and you will see the collision boundaries of each object placed in the map.
When you first turn it on, on a larger map, give it a minute to do its thing. then you can walk around and see if it collision causing the problem.
Let me know how it goes.
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