A few ghostobjects in the exact same spot- slow world update

[november]

I have a simple discrete dynamics world with just 10 ghostobjects, each with the same cylinder as a collision shape, and in the exact same spot colliding with each other. They are not rigid bodies.
The collision world was surprisingly slow to update in debug mode (0.3 second to update)
I tried switching to a simple btCollisionWorld since I wasn't using the dynamics anyway, and it was still slow (0.05 second)
To give a comparison, the time is 0.001 when the same cylinders are in different non-overlapping positions

So I was wondering, why is it so slow when just 10 collision objects overlap perfectly, even if no dynamics / rigid bodies calculations are involved ? Is there an explanation on my side ? For example the use of btGhostObject ? Or the debug mode ? Or any other reason that I might have overlooked ? Or is it just expected behavior ?