When planning a mission you've got two options regarding your waypoints.
Curved or straight lines.
If you want to go straight lines between waypoints, you can add actions.
Actions could be speed change, heading, take picture, start video, gimbal tilt etc..
Curved lines will not go through the actual wp, hence no actions allowed.
Curved and straight can be combined in same mission.
Some of the actions at the wp will be governed by the app itselves.
Speed (or speedchange) is one of them.
Location, heading etc will be uploaded to your Spark before executing the mission.
So, back to your initial question regarding speed.
When you create a mission, you set a cruise speed. This is the speed it will do the mission at if no speed change is called for at a wp.
But, you also set a max speed.
The magic here is you can adjust the mission speed with your RH stick.
Faster (until max speed reached) or slower.
If RC-AC disconnect, last known speed will be uses to the end or maybe until connection is reestablished.
But bottom line... speed is governed by the app though the RC to your bird.
Regarding low
battery, be sure to activate "Smart RTH" and "return to home" in the settings of Litchi/Mission.
Btw, Litchi is worth every penny!
Can be used with all the modern DJI drones, and even some other brands.
https://flylitchi.com/help