In the below video you see how to identify just which avatars are using all of the resources on the server causing that terrible lagg. The key is to determine their Avatar Rendering Cost. The more prims, textures, scripts etc that an avatar is runnig the more they lagg a server.
In the interest of server stability many are now looking at these things.

What Effects Your Avatar Rendering Cost in Second Life?

1. Being there +1
2. Every texture +5
3. Prim attachements +10
4. Shiny Invisible or Glowing +1
5. Size of Prim meter, axis +1
6. Bump Prim +4
7. Transparent face of prim +4
8. animated textured face +4
9. Prim Flexi +4
10. Particleemitter +16

Official Avatar Rendering Cost ARC Tutorial