Pure speculations below:
I suspect the surface lag is caused by the DF display engine first "rendering" the "ground" tile, and then replacing that with a rendering of the topmost item of the the items on top of it. It might possibly skip the "ground" tile if there's construction on it (like a bed) to render that instead. If the rendering is then subject to modifications (such as vomit, leaves, blood,...) that might add additional lag. If this process is generally correct, any kind of QS would cost the same as a regular stockpile tile. Regardless, the display engine (or its usage) have to be very inefficient for it to have trouble with the limited amount of display DF requires.
It can be noted that cages with multiple creatures in it cycle the creature displayed, which might cost extra.
@martinuzz: If you channel down into the soil of vegetation less badlands the soil below supports grass (and thus grazers), but never herbs or saplings, in my experience (and I've done that a fair bit). You can build farm plots both on the actual surface and on the channeled soil, and they support the same set of crops in both locations. I wouldn't be surprised if it would be possible to get grass on the surface as well by building on it and then tearing the building down (the mud -> soil trick, basically), although I haven't actively tried, and haven't noticed it happen, but tearing things down usually mean you did it wrong in the first place...
Regardless, your moat doesn't run the risk of getting nullified by trees, but you might possibly get a rather marginal FPS effect if you cover the moat floor with either a floor or a paved road (I wouldn't bother).