What is low input lag in monitors?
Low input lag is a feature of computer monitors, specially of the high end gaming monitors that minimizes the input lag (increased latency, sluggishness) for fast reactions by reducing the video processing time.
Usually when the feature is turned off, the HQ visuals demand additional processing time in order to show you same HQ visual no matter what frames you are trying to achieve every second. But with higher FPS (Frames Per Second) it’s not possible to maintain the same HQ visuals without buffers. Even if you try to stress the monitor to play those HQ visuals of High FPS, your monitor will bottleneck seriously. You may even encounter frame shutters or tears in unpleasant ways.
Note: Low input lag feature is totally useless when you are not gaming, and doing normal desktop task. It is only useful on higher FPS like 120 Hz or 144 Hz and only when you are playing intensive graphics.
This is where this feature is useful. So next time when you are playing GTA5 or PUBG in High settings, do enable this feature on your monitor to reduce the input lag.
Should low input lag be on or off?
If you play AAA title games a lot, enable it all the time. Otherwise there is no other way you are going to see the usefulness of this feature either turned on or off.