I can only tell you about my case...and it may not help! But...I have used most viewers from before 1.23 up to 3.0, and several 3rd party wiewers--I tried 1.0, went back to Phoenix, tried 2.0, went back to Phoenix, and the 3.0? It pegs my processors out! Down to 5 f.p.m. with water, clouds, and particles turned off! And it takes forever to log on, so I find myself doing other things now besides SL. I don't know if it doesn't run good on multi-cores or not; or 64-bit or not--but there is a problem! I downed the Firestorem beta viewer and the difference is remarkable..and I can only hope they are designing a viewer to accept mesh, otherwise SL is going to be a distant memory for me...