[unknown] the iterative solver is currently an "experimental feature"
It doesn't yet support all of the functionality present in the "Direct Solver", but does offer us a path for implementing a solver that is capable of MUCH faster simulations. Features like inertial damping require us to reformulate the algebra to work in this system, so is not available in this initial release.
Right now, where this solver will work best, is in situations where you have a system that is made of materials that are of similar, or the same stiffness (the greater the difference between the stretchy and stiff materials, the longer it will take the solver the converge)
For characters like Jellyphant, where it is one material with a single body bone, we have observed simulation times upwards of 30x faster - with our more highly constrained simulations with multiple competing material stiffnesses, we have observed more modest speed improvements.
I hope this helps!