Not entirely sure but I remember someone who is a lot more computer savvy than I, once telling me that everything above 40 FPS doesn't really matter because you won't see the difference anyway.
That certainly could be true, I'm not computer expert at all.
I wish I knew how to quantify other things that are important performance wise to me and are deciding factors in which viewer I use.
-how often I crash
-how long it takes for inventory to load
-how often I have problems attaching/detaching
-how often I don't get group notices
-chat lag, IMs going to offline even when I'm there and yo-yo slingshot walking
-how long it takes for textures to rez, on me, or others
-how long I have to stand at the hub before I can see everyone is my usual test on any viewer
I don't know how many of those are factored into or reflective of FPS but that is the only number I have that's quantifiable for comparing my performance from viewer to viewer.
For the others, I have anecdotal evidence that I crash less often or have super fast rezzing/loading of textures and avatars.
When I compare Phoenix vs. Firestorm for example. Very similar FPS, both over 40, but Firestorm takes considerably longer to rez textures than Phoenix does for me. I don't know why, it seems as though I'm the only person who has that issue. Most find the opposite.
I believe some of those are server side issues, but I can't help but notice I have more or less of those problems with some viewers than I do with others so there has to be a relationship there, even if I don't understand what that is LOL.