I think that if you hear the bit screeching, you're plunging too fast. It's an oscillating router bit and it should sound like any other router bit.
You don't want to dwell because the bit gets too hot rubbing against the sides of the mortise the bit's making and excessive heat can cause failure. You don't want to plunge too fast, (screeching sound), or you're taxing the bit and it could fail. If you have an overheated bit that gets plunged too fast, well...
I've been guilty of both but have tried to get Zen with both the 500 & 700. I think I've got it. It's different for every material and every piece but it's a "feel thing".
I've done a lot of demo's with both machines with a lot of people and some people are Naturals and have it perfectly, first plunge. Others need a little practice.
Another thing that I think comes into play is fatigue. I know when I get tired and have a lot left to do, I tend to push the limits to eliminate time in the aggregate. It can cause problems.
Trying to shave a little time off every plunge is not the best way to save time overall. Let the plunge be the plunge. Trying to save time in the 'set up and process' works better, in my opinion.
Tom