What touch latency means
Touch latency is the time between a physical action, such as placing a finger or moving a pen, and the visible result on screen. It is measured in milliseconds, which are thousandths of a second. A delay of a few milliseconds is invisible, while a delay of several tens of milliseconds can be noticed, especially in motion. Exactly where the threshold lies differs between people and tasks, so be wary of any single number presented as the limit.
Where the delay comes from
Latency is not one delay but a chain of them. Each step takes time, and the total is what you perceive.
- Sensor scanning. The touch controller does not detect contact continuously. It scans the sensor in repeating cycles, and a touch that occurs just after a scan waits for the next one. A higher scan or report rate shortens this wait.
- Signal processing. The controller filters noise, decides which readings are real touches and calculates positions. More filtering gives steadier results but takes a little longer.
- Transfer to the system. The position data travels to the main processor over a connection and has to be handled by the operating system.
- Software handling. The app receives the touch event, decides what to draw and prepares a new image. Complex apps or busy processors take longer.
- Rendering and buffering. Graphics systems often prepare frames ahead of time, and each stored frame adds delay.
- Display refresh. Once a frame is ready, it waits for the screen’s next refresh. At a low refresh rate, that wait can be considerable, while a higher refresh rate lets new information appear sooner.
- Pixel response. Finally, the pixels take a little time to change, a matter touched on in the discussion of response time.
Add them together and you get the full delay. Improving any one stage helps, but the slowest or most numerous stages dominate.
Why drawing makes it obvious
When you tap a button, you mostly care that something happens soon. A small delay there is hardly noticeable. When you draw, the situation changes. The pen tip is a physical object that you can see, and the ink should appear exactly where the tip is. If the line trails behind, there is a visible gap between tip and ink, and the gap grows when you move quickly. Your eye compares them directly.
In the same way, finger dragging can feel as if the content were stuck to the finger by a stretchy band when latency is high. With a stylus, the tip is small and the visual comparison is easier, so the lag stands out more. Many people notice it more with a pen than with a finger for that reason, although individual sensitivity varies.
Prediction and other tricks
To hide delay, many systems predict where the finger or pen is heading and draw ahead of the last reported position. This can make a line feel closer to the tip. The risk is that a prediction can be wrong at sudden turns, and the line then briefly overshoots and corrects. Some pen systems use special low-latency drawing paths that bypass parts of the usual graphics pipeline, so the first segment of a stroke appears faster. Such techniques tend to be tuned to specific devices, which is one reason two tablets with similar specifications may feel different.
Refresh rate and its limits
A higher refresh rate helps because it shortens the wait for the next frame and shows smoother motion. It is not a cure on its own. A fast screen paired with a slow sensor or slow software can still feel laggy. Likewise, a good sensor and software on a slow panel will be limited by the display. This is why a high-refresh display, a high report rate and efficient software are best seen as parts of one system.
Latency in other situations
The same ideas appear elsewhere. In games, the delay between controller and screen is called input lag, and it overlaps with touch response on devices where gaming is done by touch. Large touchscreen displays, such as shared boards, sometimes show more latency because of bigger sensors, longer cable paths and extra processing, and wireless casting adds yet another source of delay. The behavior of a pen on a big board therefore may differ from that on a small tablet.
What affects it besides hardware
- Heavy software or many background tasks can add delay.
- Screen protectors and thick glass can alter sensor behavior slightly.
- Low battery modes may reduce scan or refresh rates to save power.
- Cold conditions can slow some displays, and they can affect pixel response.
- Different apps use different drawing methods, so one drawing app may feel better than another on the same device.
How to judge latency yourself
Published figures are rare and rarely comparable, since methods of measuring differ. The simplest test is practical: draw quick circles or zigzags and look at the gap between the tip and the line. Compare devices in the same app if you can. Judge the experience over several minutes, because small irritations show up with longer use.
What to take from this
Latency is a property of the whole system, not of a single component. A drawing device that feels responsive is usually one where the sensor, software and display were designed together, and the choice of sensing method, discussed in the overview of how touchscreens work, is only one part of that.