game on iphone with bullet,i need advice on optimization

[myaue]

i am making a game on iphone using bullet,the game scene contain about fifty btConvexTriangleMeshShape and several btBvhTriangleMeshShape,but the framerate is about 15 per second.i've try using btShapeHull's utility to optimize the mesh but only gain a little improvement on performance.i read some posts about building Bullet into Release version or Debug version,but i am not sure if my bullet copy is running on release mode or debug one.the user manual did not meantion that.
BTW,i create my project on Xcode and include all Bullet's src folder into my project and compile them all together.i don't know if my Xcode project's setting has any effect on Bullet's src compilation.
any advice will be appreciable.thanks.