The fastest Street Race is 'Town Tear Up,' Southern part of the main map in near the North middle of Byron Bay city (not festival).
Change drivatar difficulty to 'New Racer' to make this significantly easier.
The race itself shouldn't take much more than 2 minutes to complete.
Video for reference of location and how quickly it can be done. I even have to rewind partway through the race when I mess up and it still takes less than 3 minutes total for the entire race.