Generally there is a lower bound because it takes a few minutes to slew to a new spot on the sky and stabilize. Very large telescopes are slower, so this will

Generally there is a lower bound because it takes a few minutes to slew to a new spot on the sky and stabilize. Very large telescopes are slower, so this will take longer, possibly 5--10 minutes. So you typically want to spend at least 20 minutes per target so that most of your time is spent observing rather than slewing. That's why there's still a lot of demand for 1-m class telescopes. There are also issues with saturation. Large telescopes simply cannot observe brighter targets because they saturate the detectors too quickly. (This is not so much of a problem for deep field objects since none of them have high surface brightness anyway, but can be a problem for brighter stars.)