The lag is - I suspect - caused by Blizzard running what amounts to a tremendous load of addons to hide all the features that was not in Vanilla. For instance golden Quest helper arrows, Sparkling resources, mouse-over quest help and so on.
From time to time these features pop up, showing that Blizzard did not remove them from the script, but added something to keep them hidden. And this “addon-soup” is at least part of the explanation for the lag.