Testing the latest (1-16-0-0), and I get about 1 fps. I have Vista, an Intel Core Duo 2.4 Ghz, 2Gb RAM and an 8800 GTS. On the main grid I get stellar framerates even in busier areas.
I am willing to undertake whatever suggestions the beta team may have to determine the cause and fix. For me anyway, this would end SL if it became a required release, so I hope we can address this, and I am willing to help.
*** Update ***
Logged out and in, everything started fine, although I started in a different area. Then I hit a "Play Media" area, and voice chat popped up. Voice chat then crashed, and SL Beta became extremely laggy on the network.