OnCommand Balance performance is mostly limited by the performance of its internal database. This refers to the number of physical spindles behind the datastore that the virtual appliance is running on. For a large install, having Balance deployed on an aggregate that spans at least one shelf is preferred.
These are general guidelines, to prevent people from deploying large Balance installs on a single local disk, or too small an aggregate for the load.
You should also monitor the Balance VA with itself, it can tell you how it is performing.