In gaming, input lag is defined as the delay between the when a user does something with an input device and when that action is reflected on the monitor.

The definition is straightforward, but the reality of input lag is much more subtle than may readily be apparent. There are many smaller latencies that contribute to the overall whole of input lag and understanding the full situation may prove beneficial to gamers everywhere.

The first subtlety is that there will always be input lag. Input lag is an unavoidable reality that can only be minimized and never eliminated. It will always take some amount of time for input data to get to the software and it will always take some amount of time for the software to use that data to display a frame of animation on the monitor. Keeping this total time as low as possible is a key mission of hardcore twitch gamers out there.

This article will step through all the different contributors to input lag, and we'll give some general estimates on the impact of each different contributor. Exact numbers will vary widely with different hardware and software combinations. But knowing where to focus when optimizing for input latency should help those who are interested.

After drilling down into the causes of input latency, we will provide a few examples of different hardware and settings in our lab. The extra twist is that we will be evaluating actual input latency using a high speed camera to count frames between input activation and monitor response. We'll be looking at three different games with multiple settings on both CRT and LCD monitors.

Reflexes and Input Generation


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  • Zolcos - Thursday, July 16, 2009 - link

    The article is logically inconsistent. On page 1 it states "input lag is defined as the delay between the when a user does something with an input device and when that action is reflected on the monitor" and on page two it has "Input lag starts from before we even react". Reply
  • DerekWilson - Thursday, July 16, 2009 - link

    i'll fix that...

    "The impact of input lag is compounded by what goes on before we even react."
  • yacoub - Thursday, July 16, 2009 - link

    The input lag everyone's most concerned with is the amount the display adds, because while all the rest is consistent, displays add a variable amount depending on which one you get. The ones that add more than ~20 ms add a NOTICEABLE amount (for most people) which takes input lag to the point that it becomes frustrating. Reply
  • DerekWilson - Thursday, July 16, 2009 - link

    Part of the point was to explain that there is a lot at the end of the chain that can significantly impact performance and it's all about the display.

    If we do consider a 100ms threshold as valid, then based on our numbers from TF2 it is clear that we would end up in the >100ms input lag range with a monitor that adds more than 20ms of lag.

    And if we can't expect a twitch shooter to come in under the mark, how is everything else going to do? Not well I would imagine.

    I did think about looking at a wide array of monitors, but I feel like that might be better suited to a more focused review of monitor performance rather than an exploration of input lag in general.
  • yacoub - Thursday, July 16, 2009 - link

    Sure but for whatever reason, all of the lag prior to the display's lag is essentially transparent because it doesn't add up to be enough to be perceptible. This would equate to your threshold.

    When using a display with little or no noticeable display lag, any FPS game will feel very responsive and without discernible latency (assuming your GPU hardware is up to the task of rendering the frames quickly enough and you're not using one of the early optical mice from a decade ago that had terrible tracking refresh rates, etc etc).

    Yet simply switching to a display with higher latency is enough to make input latency noticeable and frustrating for FPS gamers. So the key issue is finding a TN or IPS display since those panel technologies have the least input lag. Of course most panels out there are -VA based panels because they are cheaper to produce than IPS, and TN may be snappy in display response but they have a number of other downsides.

    What matters most is getting panel makers focused on IPS-based displays (or new panel technologies that significantly reduce the input lag most non-TN displays presently suffer. And hey, the more they produce and sell, the lower the production cost per unit so the better the pricing can be and the more opportunities for improved technology to be added to the IPS design.
  • ocyl - Friday, July 17, 2009 - link

    @ yacoub
    Did you read the article at all?
  • yacoub - Friday, July 17, 2009 - link

    Yes. I must not be explaining myself well, so forget it. Reply
  • DDuckMan - Saturday, December 18, 2010 - link

    While this article was great, I'm still not sure if I am better off disabling SLI to eliminate the syncronization lag or having the higher framerates with SLI enabled in twitch games. It seems to me that with 120Hz monitors, vsync (which I need for 3D) and SLI lag would not be as important as keeping the framerate above the monitor refresh rate. I don't have the equipment to properly test, so I am looking forward the the next article.
  • burner1980 - Thursday, March 10, 2011 - link

    Quote: "Input lag with multiGPU systems is something we will want to explore at a later time."

    I`m still waiting patiently and looking forward to a follow up investigation. The topic of input lag is VERY important to gamers who play FPS. I do notice it in racing games, too.
    I suggest to use true 120Hz monitors in the follow up article. They of course won`t reduce input lag, but help to reduce screen tearing and thus allowing to optimize one`s settings to reduce input lag while keeping screen tearing at a low enough level.
    I´m also courious if using a 3 screen setup a la Eyefinity oder Vision Surround using two GPUs will have an impact.
  • dmnwlv - Thursday, April 28, 2011 - link

    Impressive report.

    Regarding mouse polling rate (I may have missed it out):

    1) I believe the actual mouse input into the CPU is already calculated and the end result (of that action) already registered before you get to see it on screen. It does not wait for the GPU/monitor to finish processing before determining the end result. Hence the influence of mouse response is even more substantial if we take out the whole chunk of lag times that were included in the total lag calculation here - Derek Wilson, pls correct me if I am wrong.

    Coupled with the predictive ability of human (also reported here) to react accordingly in advance from the existing state of game situation, it seems to match and explain why it is hard to imagine a few milliseconds of difference in mouse lag can have an impact to the overall gaming experience. The brain and reaction is (trying its best) interpolating and working in tandem with the CPU than the monitor.

    2) And another scenario where the user already intended to do a series of / continuous / extended action (eg, drawing a long curve line), does the response rate of the mouse play a part in drawing the most accurate curve that the person input/intended? - Maybe Derek can help on this as well.

    Thanks for the great report.

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