Dedicated Server problem

Hello world, I’m creating a new game with Unreal engine and it uses a dedicated server. I’ve created a monster class with a behaviour tree which does only one thing: it looks for my character inside the map and as soon as it finds him, it reaches him and finally he attacks him. Monsters are loaded and performed only in the dedicated server and then replicated on the clients. In a map with about 100 monsters, everything works very well. However, as soon as I increase the numbers of monsters till about 600, the dedicated server starts going jerkily. The problem is that the dedicated server istance occupies almost 18% of CPU ( core i7 4770k) and about 1gbs of ram (16 gbs available), too little compared with the resources the dedicated server should have. Therefore I cannot understand why the gamr server instance slows down. Where is the problem according to you? Thank you very much