It depends how many stops you're going. If you get on at one stop, then get off at the next one - it probably saves you no time, or even costs you a bit of time. But if you ride through ten stops, it would probably save you a fair amount of time.
It also depends how long the stops are. The East-West train across Canada (in every way the opposite of a high speed train, so really not a reasonable comparison) has many many stops on its route, some of which are over an hour long when the train takes on fuel, food, water, etc, not counting the time to manoeuvre off the main line and onto the side line that goes to the train station, then get back out to the main line again. If it could just keep making headway as shuttle trains pull up alongside it at various towns and cities, a longer-haul trip could be cut down by many hours