One aspect of SL that is often underestimated is the importance of your network bandwidth setting. Having this too high or too low can lead to all kinds of problems, from simple TP failure to being disconnected - among other things. So it is advisable to not rely on the bandwidth stated by your provider, but to check for yourself.
Based on our experience, we strongly recommend the following maximum settings for bandwidth - unless the calculations above suggest lower values:
SpeedTest will give some indication of the quality of your connection. But another very useful indication is ping and packet loss.
You can determine these from within the viewer by accessing the statistics bar (press Shift-Ctrl-1). Look for Packet Loss and Ping SIM. Ideally, these should be 0.0% and under 150ms, respectively. The stats bar gives data based on your present location in the SL grid.
See this page for documentation on Firestorm 4.6.9 (42969) and earlier.