Performance issue with compound shapes

[vab]

We're developping a game on xbox 360 and ps3 using Bullet, and I recently noticed really bad performances when using compound shapes on xbox360. I built two test scenes to be sure : in both scene, there's almost 20 dynamic capsule shapes falling onto a ground, but with two differnt ground:
- First ground is a simple (large) box shape.
- Second ground is a compound shape made of 6 boxes of various sizes.
With the first ground, integration time is 4ms max, with the 2nd, 15ms max...
What am I missing? I'm pretty sure that with 6 simple boxes placed at the same spots, integration time would be shorter.
Is there another combound-like shape that would be more efficient for static objects?

Thanks,
Vincent

[vab]

Tested with 6 separated boxes => 4 ms max.