By default, this feature is enabled. It makes the printer slow down when the queue of already processed line segments is running low. By printing the remaining queued segments more slowly, there is more time for the queue to refill. This helps prevent the printer sitting idle when the queue completely empties, eliminating the tiny blemishes that may form while plastic continues to ooze from the idle extruder.
Normally, the queue of processed line segments is full. There are only two common situations in which the queue starts to run dry: first, when quickly printing many short line segments in small, fine detail; second, when printing over a serial USB connection where the computer sending the data over the USB is slow.
There is no practical reason to disable this option other than to see the effects of printing small fine detail quickly (120 mm/s) without it enabled.