its like this, when u make a room like so
____________
| [] |
| [|
| [] |
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[] and [ being random things added to the map for realism, such as a piller or a poster on the wall.
_-| being the walls themselves.
everything within |_- should be detail brushes, the walls should be structural. reason being, when u compile a map with EVERY brush being structrual, your compile time could take anywhere from 1hour - a week (it depends on how big and how detailed the map is.)
the reason it can take so long is because the vis(vision) has to take in account every single little brush that is structural and it can take forever with every brush being structural. so with the room i made, if the pillers and the poster are detail brushes, when compiling, the vis wont even see them there and makes the room as if it were an empty box. making it go a lot faster.
ill post screenshots later for better help