A slight variation of Rick's solution

[Mikewar123]

I did a solution very similar to Rick, except that I just used a float defaultSpeed and intialized it in the start method of Mover.cs. I intialized it as navMeshAgent.speed this way at the start of the game the enemies cache their start speed themselves, so that I can still tweak their NavMeshAgent speed in the inspector and everything will still work fine.

[Brian_Trotter]

That works. The important thing is that you have a system for setting the speed.