Mark,
What kind of entry are you making? Some kind of a ramp is considered to be the best way, better than straight plunging. The reason I ask, is that perhaps you can boost the plunge rate considerably higher than whatever you are using now (I don't know VM at all, sorry). This would be because the cutter can be fed more normally through a ramp entry than a straight plunge.
Perhaps another trick is to program each pass as a seperate process. Then each time you do this, you can set a different material top height, which may allow the tool to rapid down closer to each new clearance plane. You may not want to go to this much trouble for every cut, but if you have a very deep pocket to cut, you might want to create a new process at every 1/2" increment, redefining the material top for each, as I described.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)