In the same way you do not need to specify measures, you do not need to enter a tempo when creating an AutoGrade assignment. AutoGrade will use the tempo on the file as the tempo for the assignment by default.
If you do change the playback speed, it will affect the beginning of the score or the location you specified if you changed the starting measure location.
How changes to playback effect future tempo changes in the score
Let's say you have an assignment that looks like this, and assign all 4 measures.
If you change the Playback Speed in the assignment to 0.8, AutoGrade will apply a 20% reduction to all subsequent tempo markings. This means measure 3 will be assessed at 96.
The playback speed you choose applies to the starting measure you enter for the assignment.