For drag (or scroll) latency WALT uses a laser that shines across the touch device and hits a detector on the other side. The microcontroller monitors the state of the laser detector and reports (over usb) when the laser beam is broken and unbroken. A finger dragged back and forth on a touchpad or touch screen and interrupts a laser beam. Touch events from the pad and laser events are then processed together to deduce the delay.
A video here shows the measurement performed using a robotic stylus for extra accuracy and consistency, but the measurement also works fine with just a finger or a hand held stylus.
