Not sure about any input lag on my LG TV but it was way better than having it at 30Hz and I find most tasks and gaming is normal and responsive.
However, the thing which really bugs me about my LG 55UB850T is everytime when I turn my TV on, the text is always fuzzy, even though my HDMI settings is on PC mode which makes the text cleaner and crisper, I always have to either go back into the menu and apply the settings again, even though its still on "PC Mode" when its not, or either plug my HDMI cable into another HDMI port, select the port, unplug the cable and plug it back into the HDMI 3 port which will give me clearer text again which is a pain in the ass. Not sure if this happens on all 4K TV's or just a LG problem. This only happens when my resolution is set to 3840x2160 or higher, the text is clearer on lower resolutions.
I'm pretty sure the LG 55UB8500 is the same as 55UB850T, 55UB850T is the Australian model number.