These steps are necessary to setup the joints according to the type of drive you want to use
“Currently, two types of drives are supported: position and velocity. For position drive, set a relative high stiffness comparing to the damping, and set a reasonable maxForce. For velocity drive, set only damping and set stiffness to 0.” [joint-drives]
By the way, have you tried the tricks described in this post to speed up rendering?