Pathfinding Multiple Units - Optimization

When multiple RTS units are tasked with Pathfinding, then moving towards the destination node (if the destination is reachable), there is a noticeable lag. Each unit has its own Pathfinder script attached to it.

I understand that I am probably computing too many complex tasks at once, creating the lag. Does anybody know if there is a way of being able to compute the tasks all at once (or at least, one after another), that prevents any lag from happening.

I am using Hashsets and Dictionaries to hold important information in a more efficient manner.

I would appreciate any feedback on how to best calculate paths without causing a computer to slow down.