I've decompiled the map and inspect it with Hammer Editor, and there some mistakes I need to point it out.
Skybox - DO NOT boxing entire map within just one giant skybox, this may impact performance, also increase file size which is for rendering un-necessary space. Construct Skybox alongside with map Layout
Layout and Texture - this is also a problem as I inspecting it, while you start making a level, choose NoDraw texture first. while forming basic layout, make sure it will snap on the grid, you can reduce grid size and start adding details after the basic layout is finished. NoDraw texture tells the engine do not render lighting on it, which can reduce some file size and improved performance, only applied normal texture on the surface in playable area.
Brushes - Make sure brushes do not overlap with each other!

There is also other flaws in the map, but I won't go too many details on those, I would recommend the creator to watch some basic hammer tutorial, there are plenty of videos on youtube, and visit Valve Developer Community, to understand some basic about how Source Engine work.



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