have you func_detail'ed those railings? If not, those railings will be splitting the surface face of the wood floor into many many polygons. Func_detail or func_brush those railings.
Additionally, the railings touching the edge of the face are splitting the face with many t-junctions, regardless of whether the railings are func_detail or not. Moving the railing supports from connecting to the very edge will remove the t-junction issue and improve performance.
I'm not sure as to the actual cause of the problem, but doing these things will help in the long run, and possibly inadvertantly solve this issue.