I have a little system monitor applet running on the panel, and when the SL client is running, the CPU meter is always maxed out. I understand that the SL client is necessarily CPU-intensive, but OTOH, have you noticed this? When the client first starts up and is waiting for you to decide to either sign on, change where you will first appear, or quit, in theory, the SL client should be sitting there, doing nothing, queued up for either a mouse click or typed input--but at that moment, the CPU meter is still maxed out! What on earth is it doing to chew up all those CPU cycles while waiting on you?
Well, this morning, once logged on, I had to go away for a bit, so I clicked on the "show away"... and my avatar went limp. The CPU usage didn't budge... but then I minimized the SL window. Then CPU usage dropped to near zero.
So... that's what it takes. Minimize your SL client window to really idle the SL client.