I have to disagree. I, too, have the 2010 Macbook Air, 64gb 11" model with 4gb of RAM. Admittedly, the RAM probably makes the difference, but Lion has been running fairly smooth so far. I usually have several Chrome windows open along with Twitter, Teamspeak, Skype, Mail and Spotify, along with a few Fluid apps for Facebook and Gmail. Full screen mode makes multitasking a lot easier on such a small screen.
I haven't noticed Lion running any slower in real-world usage compared to Snow Leopard. There are a few bugs that cause some slowdowns, though. Any time I navigate an App Store page with Lion on it causes it to lock up for several seconds while it determines whether or not it's installed. I've seen this happen on display models at the Apple Store as well, though those machines are typically faster and the lag isn't as noticeable. I also notice the spinning pinwheel several times while navigating in Safari, though I use Chrome primarily and never have that problem. Issues like that will get ironed out with software updates, hopefully.
Overall, Lion is a pretty decent upgrade. Some performance optimization needs to be done on these 'older' machines, but overall it runs pretty much the same as SL did. The full screen apps alone are enough to keep me from going back for sure.
