My 60" 1/2-8 2 start whips a lot over 100ipm, but I can still run it reliably at 150ipm. My 40" Y axis whips a small amount when the nut is to one side, but not enough to cause any problems at 180ipm.
My Z axis is 1/2-10 single start, and I get 55ipm. These speeds are using a Xylotex at 24V.
You really should be using 1/2-8 or 1/2-10 acme 2 start. It's at least twice as efficient as all thread, so you'll lost a lot more power due to inefficiency.