Check your vcenter load, I don't think more mangers help that much if your the load on them isn't high. Also check the appvolumes queue settings if they are enabled, the default is to do only 5 concurrent appstacks at a time which may be too low for your environment. In my case I have 5, most applications are in 1, one for office, 1 for visio, and two specialized ones for different licensed software.
