HI FlyAgi,
I basically noticed the same on my setup, but I am not even close to 60fps :). So FPS impact hits me much earlier.
So strange thing is, that with SMP, my overall CPU utilization is "much" lower, as without SMP, even with nearly no clouds visibile. I use 11.02b1 with the performance optimizations, and yes, I know - don't use beta unless you have to, but I couldn't resist, so I didn't post here any further.
Would like to use SMP, but for me it is only usable until 25sqkm, which is not enough for my personal preferences of view distance.
Cheers,
Michael