GPS devices can't deliver realtime speed. There's always a slight lag, usually a few seconds. Whether this matters depends on the terrain. It's close enough for longer uninterrupted climbs, flats and downhills. It's not useful for the roller coaster hills in my area so I don't bother looking at my GPS devices for realtime speed while riding. However they can deliver near-realtime estimates of average speed. Depends on the device or smartphone app.