cutting speed adjustability

I currently have Enroute 5 plus for post processing of dxf files to generate g-code for our cnc router. Enroute says that they don't offer a way to slow the cutting speed down, when going from straight lines to corners, and arcs. I cut a lot of plastic, and I am trying to speed the process up as much as possible, and still make a quality part. (time is money) I would like to cut say 70 or 80 ipm on the statight cuts, and be able to slow down (20 ipm), when cutting corners and arcs, to eliminate chatter, and keep the cycle time to a minimum. Any ideas what type of post processor, can do this?
I have Centroid, cnc controller on the router, and I have centurion 7 on our knee mill, and I would like to be able to process g-code for both of them. Any ideas, would be appreciated. Both machines, can be programmed in conversational mode also, but I am new to conversational programming, and familiar with g-code. It looks like I may have to take a crash course.