alright, here is list of things you did wrong:
1. you have your sky as a single hollow box, this is first thing that impacted your performance because you have unused space under the map that was computed
2. you have absolutely no func_detail entities, and a lot of small brushwork that would need it. on top of that, your level has a lot of geometry holes, this further impacts performance when combined with big hollow box sky
read up on this for more info
https://developer.valvesoftware.com/wik ... ion#Leaves3. you have not textured roofs of buildings, players can get there easily
when you are in editor, turn off every piece of geometry from vis groups except for brushwork, you will see what is the problem