I was just browsing through junk.txt for my map and i noticed that there are around 100 brushes giving off a warning like :
Entity 0, Brush 1867: Mixed detail and structural (defaulting to structural)
i think i know where and what they are due to a drop in fps in a certain place, so i need to know what caused them and how to get rid 
Tnx
Still not sure what the problem was, but its gone