Yep, on super-low-latency connections it usually takes my alt PCs about 1-1.5 seconds for the clients to "realize" that my main has changed targets. The performance of your secondary account PCs affects this as well. My one box is a laptop that gets abysmal framerates and 75% of the time if I'm retargeting mid-fight, that one client will flay the previously targeted mob the first cast.

Not just you or your setup.