Answer to: "What exactly is doing with all of those cpu cycles III" in linden answers

In the game industry it is perfectly normal for a 3D app to use 100% CPU it is by design. A 3D game like app is working in real time and try to draw as much frames as possible ( one frame is each time your screen refresh).
Since the content of the scene vary in time, the time needed to draw one frame is variable, in heavily detailed places, you will get very few frames drawn in one second (thus the choppy feeling)
in a lighter zone, the frame drawn per second can go well over 30 on a good computer, and the game feel like it react fast and smoothly.
This scalability cause the game to try to use as many resources at possible.

It is clearly a wrong approach for a spreadsheet program or a painting program, but for games it is perfectly normal and inevitable behavior.

I totally agree. I'd rather complain if I bought a fast PC for SL only and the client software didn't make full use of it. Usually I only run SL and want the best possible performance. But I know that I can still open a 140 MB PSD file while SL, Photoshop, Poser, Excel, my browser and WinAmp are running, if needed. I couldn't do that with any other newer 3D game.