How are you currently turning them all on/off at the same time? Do you have them grouped using a scene and you are activating the scene?
If the devices support associations you could try setting them up using this ZWave tweaker driver I ported from ST. What this would do if setup correctly is when one plug turns on or off, it would send a command directly to the associated devices to also turn on/off. This could possibly speed it up a little but if the mesh is causing the slow down it may have the same issue.
