With 2 recent LG webOS tv's ( 55UB850v and 55EC930V ) I had these problems as well. After an hour, both tv's remotes were responding much slower. Changing volume resulted 5-10 seconds in actually changing the volume. After a few hours it is even slower or no reaction at all.
But...., I found the cause in my situation. Someone mentioned turning Simplelink off and that might work as well but for me the problems are gone when I disconnect my HDMI cable from my Raspberry Pi which can be controlled by HDMI-CEC / Simplelink as well. This combination worked fine on an older Samsung tv.
I hope someone else can be helped with this information but in the first place LG should seriously take a look at their "standard" Simplelink / HDMI-CEC implementation !!!