To share my personal experience,
for those of you experiencing sudden drops in speed, it is due to the low voltage cutoff.
When i first installed my HW, I set the cutoff to 3.4V/cell, which works out to be 6.8V cutoff. What happened was, when the turbo kicked in, the amp draw was too large, causing a voltage drop that dipped below the cutoff threshold. That's when you experience the sudden drop in speed. As Cobra mentioned this also happens if you have weak batteries.
I have since stuck to 3V/cell and no longer have any issues