We've tested for stability when adding and removing nodes, but haven't compared the time-to-balance of tempest versus the default balancer. Because an ES cluster remains fully functional (you still have access to all data) while a rebalance is in progress, we chose to optimize for resource usage rather than time-to-balance. There's not really even a good way to compare the balancers' time-to-balance, since they're both highly configurable (range_ratio and iterations in tempest's case, the 4 balance weights in ES's case) - default values probably aren't "equivalent" in terms of time-to-balance, since it's a very minor concern compared to resource usage and stability.