Fixed Timestep vs Solver Iterations

I'm working on a game where I have multiples blocks connected by joints. In this scenario, to solve the "spring joint" problem what I should tweak, the fixed timestep or the solver iterations?

For example, I'm using 0.005 for timestep and 48 for solver iterations, but using 0.01 for timestep and 96 for solver iterations I got a similar result. So, what could be better for performance?