@ Lil Killa - Detail brushes are not ignored by the compile process. However, when the VIS portion of the compile process finds a detail brush, it doesn't block VIS. Meaning that anything behind it will be drawn.
If the chairs are actually slowing down the framerate, making them detail brushes won't do a whole lot to help that. It should speed compiles, however. The engine still has to draw all the triangles that make up the chairs. Speaking of which...
@ Mr. Chopper - How detailed are these chairs? And is there anything else in that part of the map that could be causing the slowdown?