You are correct to worry about then input lag of this particular model. If you are into fps, fighting games, racing games - most 4k 2014 models are not for you. Check displaylag.com and you wanna look for something 40ms and below for input lag.
Now for movies, you might be satisfied despite the fact that this is sammy's entry level 4k display.
When i went tv shopping last november, my choices were narrowed to this model and the sony 50w800b. I chose the later despite it just being a 1080 display.