sorry to say, but that is BS. I'm having the same issues with a computer that can run Crysis 2, Dragon age 2, GTA4, etc at thier highest graphical modes, I can run SL in ultra mode with shadows and no lag (before these new clients started rolling out) after a completely fresh install of windows and the necessary driver updates, The SL Client lags to hell on the login screen pushing 1FPS or lower.
Don't tell me it's my internet connection. The page it shows, THAT is a webpage. The login input boxes, the menus, the minimize/maximize/close buttons... Those are part of the program. It is the Program that is causing this lag. IF I can get through the lag and sign in I sometimes have to wait any amount of time between 5 minutes and a half-hour before my framerate suddenly skyrockets up to 70 and I can use ultramode again with NO lag at all. (AGAIN... This is AFTER I log in now. Are we keeping up? Good.)
So for it to suddenly "Wake up" and give me back my framerate a half hour after I log in is unacceptable. LL needs to fix this issue ASAP. It's not our computers, It's not our internet, It is NOT our drivers... It is LL's software that they are releasing that is full of these bugs that is causing my SL to become non-existant.