In short, a bug in the viewer causes textures beeing reloaded repeatedly without any necessity - generating a lot of lag and system usage.
Now it seems that the reason for this behavior was found by a resident. It's very interesting, what he says about this bug:
"Imagine now, 50,000 people logged in, all forcing continual downloads. What a thought. It is at this point that I wish to suggest that the "overuse" of resources in SL, was a distributed denial of service attack brought on by faulty SL viewer code. I think that any observations and decisions made about the system overloads and SIMs in the last few months (well, for as long as the bug existed in the 1.20 viewers) are invalid because of this bug."
I think under these circumstances it's highly recommended that the Lindens carefully re-think their decision about OS SIMs. It's not the SIMs that are causing heavy load, it's a viewer bug!