Basically it boils down to this.
If you traverse too fast for the speed of the timber rotation you are trying to cut a screw thread.
2 ways to fix this. Speed up the timber rotation or slow down the traverse.
There is a balance between traverse and timber rotation speed.
In the case of metal lathes our teacher always said, "slow speed, slow feed. High speed, high feed but a lighter cut. For finishing, high speed, light cut, slow feed."