(For those not familiar with Second Life, every object that does something has a little program in it. Every chair you can sit on, every door you can open, and every clothing item where you can change colors has a small program written by some users. Most of those programs are almost always idle. But there's a tiny amount of CPU time consumed on each frame for each idle program, about 1us to 2us in the Mono implementation. A region can have 10,000 little programs, each eating 1us on each simulation cycle, 45 times a second. This adds up.)

The amount of heavily custom scripted HUDs required to do things in Second Life seems pretty insane last time I checked it, one for a head, one for a body, one for animations etc. I'm surprised the viewer doesn't have a way to "dock HUDs" where they can be activated/deactivated with one click when not in use in viewer managed regions.