When mapping with GtkRadiant and Netradiant, there are technical differences between structural and detailed brushes. Structural brushes play a significant role in generating the BSP tree, which optimizes rendering and collision detection. On the other hand, detailed brushes, while visually important, don’t directly contribute to the BSP tree. Having an excessive number of structural brushes can potentially impact performance during the compiling process. As a general rule of thumb, optimize your map by converting structural brushes that do not have to be structural into detailed brushes. For example, creating a table using structural brushes isn’t efficient. Use detailed brushes instead.
Sometimes it is hard to find tiny, hidden structural brushes that should be made into detailed brushes. This video shows how I do when I find them. I hope it will be of help to beginner mappers: