It appears we have two methods for setting initial mission speeds. Both will
From the Solo App, you adjust the overall performance in the settings. Like you said Turtle to Rabbit speed. So if your setting is on Rabbit get ready for the ride of your life, scalded ape type of stuff.... Further if you set a WP in Tower to Change Speed and your setting is Rabbit, nothing is going to change...still full speed. If changing the app settings is your method for speed adjustment it would make sense to set speed to turtle. You should then be able to increase speed within a mission WP by invoking a Change_Speed.
Haven't done the following just yet, but it makes the best sense to have set if running missions. It disregards your settings above, as this is a WP Navigation setting...so it establishes the default speed for WP mission speed only. See this...
Solo V1.2 release | Page 7 | 3D Robotics Drone Forum Maybe
@Jubalr can chime in further to provide a workable setting for WPNAV_Speed.