UPDATE
When dealing with mass scale of crowd I think it is better to make Ai do things in Bp instead of BehaviorTree I tested this with 3000 pawn when main job is done and I used BT event even it still keep ticking so my fps rate not improved much, it is 22 - 24. When I make the ai behave in their own Bp; when the job is done fps gone up to 26 - 28.
I don’t think BT can be paused or stopped in this version and when you try to change pawns ai behavior class it will not responde after you change it, or it is not updating the changes. I’m not sure but will update it here when found a better way!