Are you on the latest FS viewer and a Mac user? Cos lag has never been the same since I upgraded to FS4.6.
I literally cannot type when I'm in a sim with a couple of avatars.
This is due to a bug called Cocoa or something. I know nothing about these things and that makes it worse trying to find a solution.
The kind people at the FS chat group advised me to upgrade to Mavericks, so as to curb some of the problems I was experiencing but I was warned it won't solved the lag entirely.
They were right.
In short, my SL experience is now being really compromised.