Its recommended by Blizzard Technical Support that you do NOT change this setting from its default unless you've been instructed to do so by a Blizzard Tech Support rep. Changing this setting can make WoW do funny things, and can bog down your system while you're playing WoW.
From patch 1.11 patchnotes:
Optimization code known as "M2Faster" is now enabled by default. M2Faster can improve performance in crowded scenes when "Vertex Animation Shaders" is turned on. This function can be turned off by setting M2Faster to "0" in the config.wtf.
- Reduces the unique number of vertex shader permutations that we will choose to use in rendering
- Sorts the drawing of items in the scene a little differently to reduce the number of times we re-program the vertex shader hardware.
- Integer value between 0 and 3
SET M2Faster "1"