-learn how to use hint/skip textures, and
don't "func_detail" everything, especially the walls. tons of youtube-videos on this
-also, you can add a fog_controller(i think thats the name) and sett the max view(draw) distance at 3000 or something.
-make sure you set the water in you're map to change to "cheap water" at a short distance.
-and every prop dont need a max view(draw) for more than 1500-2000. 1500(cars, doors, windows, plants) for static and 1800 for phys objects(sofas, vendinmachines), is nice.
the most important is the hint/skip textures
https://www.youtube.com/channel/UCiKV_fEUKDc2IxTvTTQLegw <-- this guy knows what he is talking about
https://www.youtube.com/watch?v=O44wZitRJwo&list=PL1E042178C0497364&index=20 <-- this and the next 5 videos is about optimization. Again, fixing leaks and Hint/skip is the most important and most effective.