R_Speed reduction through mieter cuts can greatly lower your
  wpoly count. In the image to the left, Cube A has 1/3 of the visible  faces that Cube B has and yet has the same amount of brushes. The
 only difference between the two is that Cube A is mieter cut.

      So, what is a mieter cut then? Quiet simply, its a 45 degree angle.  You can achieve this thru either Vertex Manipulation or by using the  Clip Tool, which ever you use is fine.

      This is one of the easier ways to lower your level's r_speeds and
 it should make a noticable difference if you mieter cut all your walls
 and roofs. Even if your level is doing fine in the r_speeds department
 you should get used to doing this as much as possible since lower
 r_speeds never hurt eh ?
Cube A
Mieter cut
(6 brushes, 6 faces)   

Cube B
Non Mieter Cut
(6 brushes, 18 faces)

 How To ...

Vertex Manipulation:
 Select your brush and click the vertex tool (SHIFT-V) and simply
 select a vertex and move it so that you get a 45 angle (as shown in
 Cube A's wireframe (bottom diagram).

Clip Tool:
 Select your brush and click the clip tool (SHIFT-X) and click and drag
 a line from the outside corner (point A) to the inside corner (point B).
 This should leave the small triangle at the end a dotted red line. If it  isn't then click the clip tool to cycle through the 3 clip modes until the
 small triange is a red dotted line. Hit ENTER to clip the end off.