You perhaps mean "place" and not "tap"? the harpist alternates hands on a trill so the placing occurs while something else is going on and doesn't delay the trill...
With a harp, the task is more complicated. You need not only to tap the strings but to pluck them. A flute might be good (as long as the build-up of the tone does not take to long), or if it is simply for the matter of motor speed: a table. [A clavichord action comes close to that.] susan allen wrote:I wonder if the "fastest trills" might be performed on a harp, since there is no mediating mechanism at all, no action, not a thing between the fingers and the strings.....