It was very easy for me to see the lag and time it by how long it took for the letters to appear in chat. Unlike before when they would appear instantly, there is a .25 second lag time before the letters show up in chat. I can actually see the words I type appear one letter at a time after I've already finished typing the word (yes I can type that fast when warmed up).
I went ahead and tried Diablo 3 as well to see if the input lag was present there, but it is not. It is limited to WoW 5.3. This was not present during my PTR test runs.