Input lag is the period between input and output, not just the time to poll the input. E.g. on 250 Hz standard mice just polling the mouse will average 2 ms input lag with 2 ms jitter and that's before you've even done anything with the input. If you don't have a high refresh rate VRR gaming display and don't want tearing the same repeats, e.g. 60hz would be 8.3 +- 8.3. That's halfway to 20 and we haven't even gotten to delays from the game code where you can choose things like triple buffering for higher FPS at the cost of another frame of latency. Input lag can also include output to the monitor and delay from the monitor depending how it's being measured.
In this case it's more about the rendering pipeline input lag so the USB polling delay and the monitor output delay aren't counted, the swapchain delay to prevent tearing or trading latency/FPS are though and then you have to add in your actual game still.
In this case it's more about the rendering pipeline input lag so the USB polling delay and the monitor output delay aren't counted, the swapchain delay to prevent tearing or trading latency/FPS are though and then you have to add in your actual game still.