Generally speaking, threading at a higher speed will give cleaner threads, but finding the "sweet spot" is something that each person has to experiment with. There are many variables that play into threading such as material type, diameter, cutting tool type and size, DOC, thread pitch, tool stickout, threading between centers or not, rigidity of machine...the list goes on and on. The point is there is no one rule that gives the same results for everyone so you really have to use trial and error to you advantage. When you do find something that works really well for you, take pictures and notes and record those settings somewhere to refer back to. You will most definitely tweak things from there, but at least you'll have a solid starting point.
One disadvantage to threading at higher speeds (away from the chuck) is that your reaction time is severly reduced so you want to become very familiar with your machine. You will have to learn how to engage the lead screw and half-nut at the proper time on your thread dial, and each machine is set up differently so pay special attention to that. You can always adjust your speed up or down to your comfort level.
Again, this is only a technique you should use when threading away from the chuck. The most important thing is that whatever operation you're doing, just take your time and be safe doing it.
Hope this helps Ben.
