It's likely the additional interpolation or upconverting required to go from 2560x1440 to display on 4K. Higher lag too likely. 1080P to 4K is fine as it's just quadrupling the pixels which can do 60Hz.
Time to overclock your GTX1080 a bit perhaps. I'm running an OC'ed GTX1080 and BF1 is...
Text looks darn good in Game mode as I can barely tell the difference. I bought this for 4:4:4 PC mode but ended not not using it. How ironic.
Nonetheless. This thing rocks as PC monitor. 4K gaming is mind blowing coming from 1080P.
Is there a response time difference between the various models I wonder, 6300, 6290, and 6270. Can't find any info on the 6270 as it's pretty new it looks like. Asked Samsung online chat and was told it's identical to the 6290 but with 1 addition image control feature I can't recall the name.