Yeah, it's Immovable, but it's still collidable. I ran into this, too — I ended up using the Ants profiler trial to figure it out.
Even though it's no longer able to move, it will still generate all the collision points for it - so other objects can collide with it.
The problem in my case was that I had jammed a couple of building objects into my TriangleMeshObject ground. Same problem as you — much slower speeds when those objects were enabled.
To fix it, I added the ground object's CollisionSkin to each of the buildings' Skin's 'Noncollidable" list (I think that's what it's called.)
After that, the ground and buildings will no longer be tested against each other for collisions.
I think you can register a function with a skin's CollisionCallBack event to do the same thing - if it returns false, then you will get no collisions. So, in the function, you check to see if the other object is the ground (Or member of terrain class, or member of same team, or whatever) and return false if so.
Sorry if this is a little illegible, I'm rattling it off at work before I run out in…1 minute. =) Hope it helps though!