I noticed those tiny missing textures as well. A few of them were fixed after I mounted L4D2, but not all of 'em. Maybe they're custom textures that weren't properly packed into the map file? I think it'd be nice if the dev said what games need to be mounted in the description.

As for the FPS drops, that's just due to Source's ancient optimization techniques mixed with the fact the map has no env_fog_controller to help cull anything. It's bound to happen on any massive map with lots of detail and no fog to hide it.

I reccomend using the FPS Saving Fog addon https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=2925774481 and setting the distance to about 4096. Good balance between visible distance and decent FPS. Make sure you also activate it using the command "fpsfog_active 1" since it isn't activated by default.