I play on Cannith for the most part, and play odd times depending on the day, usually 4am to 7am Pacific on a workday and anywhere from 2pm to 2am or later on a day off.
I've noticed a little extra lag, but that's most likely attributed to the convergence of bodies around the Marketplace and House K near the crafting areas. I've not noticed any game altering lag during quests to be honest.
But I have noticed that my client decides it needs to close when I enter some areas. Happened most recently this last Wednesday. Was heading from the Marketplace into House K when I crashed to desktop.
Come to think of it, there might be something going on with House K in general. The other recent crash happened when going from House K proper into the Red Fens a week or so back.