Unfortunately without some form of level streaming or other method to reduce the amount of data being rendered, the engine will run slowly as you have mentioned. You can potentially use static meshes instead of bsp’s to alleviate some of this (and eventually bsp’s should be converted to static meshes to reduce their resource cost anyways), but otherwise this delay more than likely will continue to increase as you put more information into the editor. Another option would be to lower the LOD on your meshes the further you move away from them until they are culled out completely to try and reduce the information present all at once. Turning off “update bsps automatically” in the Editor preferences window and realtime in the viewport may help alleviate some of the slow responsiveness. Finally, you can use sublevels to hold data such as lighting, static meshes, etc. so that you can turn those sublevels off when not working on them, reducing the amount of data being rendered while you are in the editor.