Units do not make lag unless they cause such poor performance the frame rate drops below 15 where stuff becomes unresponsive.
1. Pathing blockers should not consume any resources at all as they should only alter the pathing map (if blizzard implimented them correctly).
2. WC3 only updates mini move pathing once per internal frame per player so giving 1 player a lot of units (especially if they have large collision) will result in very bad movement patterns (described as stop start movement chains).
If you are making an RPG, consider only creating the units when a player is potentially able to fight them. There is no need to have every boss on the map at the start of the map if each one needs the player to go through a dungeon. Instead you can create them if and when a player starts going through the dungeon.
This can be viewed as a more efficient loading model and is used in most commercial games to keep the amount of resources needed down.
An example in real life is in Diablo II, where the dungeons are generated as you enter a floor but the creatures populating them only when you get near to rooms.