There are different ways to determine speed. A probe sent to the sun has to go to Jupiter to null out the angular momentum it got from Earth. The solar Probe + will tear past Sol at 200 km/sec. Now you have Voyager, which did gravity assists on each of our four gas/ice giants and completed the grand tour after all now that pluto isn't a planet anymore. New Horizons started out more quickly than the Voyagers, but will never catch up due to only passing near Jupiter. So it all depends on what you mean by speed. A comet can pass fast near the sun--but it will slow down if in an orbit as if moves farther from the sun. Russia uses similar Molniya orbiting craft due to being too far north for many geosynch sats to service.
I would consider something fast if it can go extra-solar, and not count whizzing about in the inner solar system.