Define "going slower." What does it mean?
Longer elapsed time before reaching South Pole
We could find an arbitrarily long path and define speed = (distance between poles)/(elapsed time.)
This definition gives one value of speed for the entire trip and does not depend on the layout of the path.
We could go slower under this definition by flying endlessly at night at a constant latitude. Lower path speed.
This means speed = ds/dt where s is path length.
Here speed is an instantaneous value.
With this definition we could go slower just by stopping to rest for a while.
That is, we could make our (instantaneous) speed zero. That is very slow.
Also define "best path."
I think of best path as most efficient - shortest path or earliest arrival.
Going slower seems to ask for the path that will tire him the least.
So, instantaneous speed, average speed, longest transit time, shortest path, rest stops for the reindeer to regain strength, least time between houses so children don't have to wait for their toys, lowest to the ground so everyone can hear HO HO HO clearly ... there are a lot of things that might make a path "best."