Forgot to ask, what do you mean by "Detail Brushes"?
A quote from another site on that:
"When you create a brush, it is by default structural. This means that not only will it be drawn on the screen and give players a bloody nose if they run into it, but it will also stop the Quake2 engine itself from rendering objects behind it. Quake2 stores a lot of information in active memory that it isn’t actually drawing at the time. To avoid loading issues, the engine keeps track of a number of things that are "just around the corner." Any brushes that are structural count as solid walls in the engine’s mind when it is deciding where the corner is. Detail brushes, on the other hand, will be drawn on the screen, and will cause the player to clip against them (bloody nose and all that), but will not stop the engine from thinking about what is behind them.
In theory, you might want every brush to be structural. The logic behind this is that the less the engine is thinking about, the faster it must be running. However, this theory doesn’t pan out. Its logic holds true only if players are holding perfectly still. Once he moves, the more pieces the level is broken up into (by the excess number of structural walls), the more pieces have to be loaded in real time. And because each of those pieces has to be calculated when you compile your map, in-game loading times, in-game re-renderings, and map compilation times all shoot through the roof. It takes well over ten thousand times longer to complete the VIS process in compiling if you have a small map that is entirely structural. As your map size increases, your compile times grow exponentially.
The other extreme isn’t any good either, though. If you were to make every brush detail (except for the outer walls - more on this in a minute), you would have an abysmal frame rate, because Quake 2 would have to render everything at once, no matter how little of it you were actually seeing.
The solution to this problem is to make every brush that serves as a wall into a structural brush, and every brush that represents something less solid (say, a table, a book case, or our crate) into a detail brush. This will mean that everything in a given room will be rendered by the engine at all times, but, since everything in that room is loaded already, running around that room won't require any reloading. "
