When I've done some HDPE turning, I've used Iscar cut-grip groove/turn tools to good effect. Machining from the chuck outwards is also a good idea.
The groove turn tool permits a very aggressive feedrate without producing a shabby surface. You could probably turn at .03" to .04"/rev. The broad flat nose of the tool with proper chip breaker geometry, helps keep the chip on top of the tool instead of sneaking around the corner like happens with pointy tools. Even CNMG is too pointy.
This tooling also gives you the option of plunge roughing, which may work really well if it produces good wound up coils, and this automatically breaks the chip when you move to the next position.
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)