Slowdowns Caused By Physics??

i'm getting a severe slowdown of the simulation, and i'm not sure why...

the problem occurs in both in T3D and the older TGE (stronghold demo)...

i've added a waterplane object and some terrain... the viscosity on the waterplane is set to 444...
i'm using a buoyant physics object (submarine), and impulses to move it... and set transform() to turn it...

the problem is that i get a severe slowdown when the buoyant object turns near terrain or other collidable objects...

with the little bit of confusing info i've given, is there anything that is apparent that may be causing the slowdown...

THANKS

--Mike