Try this... and you can do this w/out having to log in.
1. Load up SL into the log in page, but don't log in yet.
2. Go to the "me" tab up at the top left.
3. Go to preferences.
4 Go to graphics.
5. In graphics, press the "hardware" tab on the lower right.
6. Uncheck the VBO (vertex buffer objects) settings.
7. Then press ok and log in to SL.
When I bought my new computer, I was having problems with severe sliggish movments, or not really moving at all until unchecking this one thing. Now I can move smoothly. Also, I keep my graphics settings on low and customize whatever I want from there. I have my draw distance at 216 w/out any probs when before it was causing a lot of lag with me at anything over 90. lol. Not sure if this will solve everyones difficulty with slow movement, but it sounds like this may be the overall problem.