Adding cargo to truck makes game not fluid as before

Hi, I'm building a cargo truck game. I have terrain and a truck and everything is great. But when I add cargo (box with rigidbody and box collider) on truck cargo space I can see some hard noticable stutterin of terrain when I drive and make a turn. Simply I can see that game is not fluid as before. Cargo is lowpoly, takes 1 draw call and 100 tris / 100 verts, interpolate is on. Did anyone experiance this before?