Dynamic NavMesh Kills Performance

I have 180 AI on my map that move around randomly and are spawned in and out within a certain radius of the player automatically. I want to create a huge map with world composition and so need to use the dynamic navmesh with a navigation invoker on each NPC. However generating navmesh at runtime is having a huge impact on performance. Is it possible to have that many AI with a dynamic navmesh or am I going to have to re-think my design?