The design, i.e what you see, is only relevant to the speed at which a low end connection will load the page, and how easy it is to see, and find the informtion you need.
The back end, which is Navitaire, is the core of the booking engine, and that is where the problem lies. Until that is fixed by the supplier - the improved all singing and dancing version obviously not as stable as the old functional system (c.f. Windows) - it will have its problems.