You may be at least vaguely familiar with monitor terminologies like refresh rate, response time and contrast ratio. However, one specification that is often mentioned but not quite as well understood is input lag. Response time refers to the operations taking place onboard the monitor itself, i.e., the measured time for a pixel to shift from black to white and back to black (or gray-to-gray). Input latency or lag refers to the amount of time it takes from when it receives a signal (when you press a button on your controller or move your mouse) to when that action occurs on the screen.
High input latency is a surefire way to ruin your PC gaming experience. It could present as an annoying delay between the time you move your mouse, and the on-screen character parrots those actions. This could also present itself when typing, with letters and symbols appearing on the screen at a delayed pace, increasing the propensity for typos.
When it comes to input latency, there are a lot of different factors at play. One aspect involves the peripherals you're using when gaming on a PC. While wireless keyboards and mice give you more freedom to move around without hindrance, a wired solution will always provide lower latency (if ever so slight). This is why professional gamers will often stick with the best wired gaming mice to maintain any possible edge over the competition. On the other hand, Bluetooth-based peripherals are well-known offenders regarding higher, unwanted input latency.
We must also consider the monitor's onboard processing unit responsible for enhancing the image on the screen. With the best gaming monitors available on the market today, technologies like AMD FreeSync and NVIDIA G-Sync don't come without a slight performance penalty, although some monitors are better than others. Support for High Dynamic Range (HDR) and features like edge sharpening and adding crosshairs (among other alterations) can also come with a performance hit, which adds to the overall input latency. However, cable interfaces like HDMI and DisplayPort don't add any noticeable latency to the equation.
An input lag of less than 10 ms will be preferable for hardcore gamers. However, even with a latency of 15 to 20 ms, it shouldn't be enough to cause any severe lag issues for gamers when they press the WASD keys and furiously tap on the left mouse button to move in for a sweet headshot in Fortnite.
This article is part of the Tom's Hardware Glossary.
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Brandon Hill is a senior editor at Tom's Hardware. He has written about PC and Mac tech since the late 1990s with bylines at AnandTech, DailyTech, and Hot Hardware. When he is not consuming copious amounts of tech news, he can be found enjoying the NC mountains or the beach with his wife and two sons.