I've run several mapping missions. I create the mission in MP first, upload to Solo, and then download to Tower for final inspection. I don't remember where I got it from, but 5 m/s (~11mph) is what I run the mission at. Word of caution, MP speed is in m/s while Tower is mph. So, if you follow my worflow, you need to verify the correct speed in Tower.
In my experience the speed change happens immediately after the prior waypoint. In the example 5r49cfu provided, I would see the speed change immediately after Solo hit WP1.
I use Geosetter to tag the images.