I'm having exactly the same issue with 1.6 too, my FPS dropped from around 40-60 to 10-15. My processor is Intel i7 and i have 8 GB of ram, but unlike you i have Windows 7. I figured it was something to do with the new launcher because even if I downgraded minecraft to 1.5 (in the new launcher) I still had the same problem.
Gabrielius Slakaitis
I'm having exactly the same issue with 1.6 too, my FPS dropped from around 40-60 to 10-15. My processor is Intel i7 and i have 8 GB of ram, but unlike you i have Windows 7. I figured it was something to do with the new launcher because even if I downgraded minecraft to 1.5 (in the new launcher) I still had the same problem.