I don't see any lag on a 2011 core i7 iMac with stutter when using the Mission Control and I opened like 20 tabs on Safari and a whole bunch of other applications open too hmm. What spec is your 2010 iMac?
Even if you have a thousand tabs open in one Safari window, it's still only one Safari window Mission Control has to render. That won't really affect performance unless Safari takes down the entire OS.
I had the same issue with OS X Lion at first. Apple fixed it later on and everything was buttery smooth again. Undoubtedly they screwed something up again.