New to this thread.
It's highly likely it's software.
One good example of this is the Nexus 7 FHD 2013.
It was really unresponsive, to the point where I would not use it.
People started tearing into them and saying it was physical, reseat the cable, etc., and said it worked.
They lied.
I never touched mine, and once updates started rolling out, and 4.4.4 and 5.0 hit, it was like a new machine all together.
I just received an F-500L engineering sample LG G4, and it works great, still has the packaging plastic on the screen, I'll peel it off eventually and get tempered glass or a proper screen protector.
Unit shipped with Android 5.1 build LMY47D, software version F500Lx10d_us.
I'm pretty sure this is the latest and greatest until 6.0 comes out.