Touchscreen latency is the time lag between an input on the screen and the device reacting to it. It affects the user experience directly and a high latency is what causes the perception of lag on devices.
In a test conducted by a company called Agawi, we get to see how some of the current smartphones on the market fare when put in front of a high speed camera shooting at 240fps and a custom built measurement device called Touchscope. They compared the iPhone 5, the iPhone 4, the Galaxy S4, the Lumia 928, the HTC One and the Moto X. The result, Apple’s devices being way ahead of the rest.
As you can see in the next graph, the iPhone 5 (not the new 5s, mind you, but the last year’s model) is way ahead of its Android based competition. It’s nearest competitor is the iPhone 4, released in 2010, and the nearest Android based rival being the Galaxy S4, with over twice the latency.
This is one of those things that is never mentioned in spec sheets but has a real impact in everyday performance. It’s also why in subjective tests the iOS devices feel much smoother and responsive than Android devices.