Oddly, Starmap doesn't show up when one searches for "astronomy" on iTunes, but only when one searches for "planetarium". I think I'll drop the developer a line on that!
I wonder what's making it sluggish... that doesn't bode well for astro software on the iPhone unless it's an obvious programming bug or a lack of magnitude controls (perhaps it's trying to render *all* stars w/o mag limits). Come to think of it, though, when you say "sluggish" what do you mean? On my Sony Clie SJ33, Planetarium is simply wonderful, but it does take several beats when changing directions, say from North to South. I can well imagine that Starmap would be slower than GoSkyWatch as it is displaying a small number of bright stars to show the constellations!
How bright is the screen when in these programs? Will they ruin night adaptation?
I had to give up on a Palm T3 and a Sony Clie TJ37 for dark work--either astronomy or ebook reading-- because the screens are far too bright even at lowest setting (and rubylith makes the screen indecipherable).