So... the new RC client was released the other day, and I had to upgrade to it. I signed back on... and found SL running like molasses at Point Barrow.
Signed off and back on. Same thing. Teleported. Same thing. 3.2 frames per second. I gave up in disgust and went about my business.
Thank goodness it must have been some problem with servers or communications. When I signed on later, things were back to normal.