Viewer and Stats
Have you ever gone around a region in second life and thought OMG i cant move (LAG)? There is a way to check to see if it is your viewer or the region(sim) your in, Click View>Statistic Bar this will bring up a panel on your screen below.
http://wiki.secondlife.com/wiki/Statistics_Bar_Guide
The link above will give indepth information about what each part does on the picture above.
The Main ones to watch are (fps) and Packet Loss, Lower the fps= slower Second Life, Higher the Packet Loss = More Data being lost between you and Second Life.