Try a higher number for the debounce. I use 10, some can use a lower number, yet others have needed to increase it.
Just depends on the signal. If the rpm is fluctuating too much Mach will sit there a long time waiting to get a consistent rpm.
At higher rpm's the DRO can't update fast enough to shows the rpm for each time an index is sensed.
You rpm should be within say 1 rpm under no load at spindle. I use a Halls affect sensor along with small magnet and works a treat.
Try this......... dry run with a few different rpms settings for threading and leave the index setting at 10. Say 50/100/200 etc and see if the thread path will start. Some sensors are not good at higher spindle rpm,s .
As to quality of signal one can view with an O'scope but most don't have one.
RICH