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In order to measure efficiently the drag latency on touchpads, the Chrome OS team has handcrafted a USB device named Quickstep. 

The video below shows you it consists of a laser that lays across the touchpad and hits a phototransistor on the other side. Each time the laser is broken/unbroken by a finger in-between the sensor and the laser, the device reports it immediately over USB.

Learn more about Quickstep by reading the official documentation at