1. A system for testing user interface software for time lag in actuating a visual prompt responsive to user manipulation of a user input device, the system comprising: a display unit defining a canvas for displaying image frames; a root capturing unit executable to capture user actuation of the user input device as at least one root event at a root software level, an operating system operating at the root software level, each root event being captured as a series of time-displaced samples of input device actuation; a canvas capturing unit executable to capture processing of the root event by the user interface software as a canvas response at a canvas software level, the user interface software operating at the canvas software level for user interaction with an application, the canvas response being captured as a series of time-displaced image frames; and, a test analysis unit coupled to said root and canvas capturing units, said test analysis unit executable to determine a parametric difference between corresponding ones of the root events and canvas responses, and to determine a degree of visual responsiveness for the user interface software based thereon, said test analysis unit thereby discriminating portions of the time lag introduced at the canvas software level from portions of the time lag introduced at the root software level.