Snow Leopard vs. Lion performance: what are the differences?
There's very little difference.
Check out this cnet review it shows the performance of applications like Photoshop, Call of Duty 4 etc. on a 27" 2011 iMac running Snow Leopard & Lion: http://news.cnet.com/8301-17938_105-20081987-1/snow-leopard-vs-lion-performance-head-to-head/, which concludes:
For the most part, we found only minor speed variations between the two operating systems.