Am I the only one who experiences a terribly slow internet connection when connected to wifi?

There are times when I tried to open the same website on two different computers and the Air lags a lot. I only get to open the site when I disconnect and reconnect to the wifi access again.

Are you running OSX Lion? I have had sooo many wifi problems since I updated to Lion. Google OSX Lion wifi and you'll see it's a big issue. There are several workarounds (none of which seem to work for me) but if you're running Lion you might want to give them a try.

unlikely the Mac. more likely the router, router channel selection and whether it's connected to Wireless A B G N and 2.4ghz or 5ghz.

I have an MBA 13", and in my experience, the WiFi is faster and more reliable than with any other laptop computer I've used on a regular basis, also at locations with a weak signal. The larger model might be better equipped though. There isn't much space for stuff in the tiny 11" model.

Check your DNS settings on the MAC and the other computer. They could be looking at different DNS sources, which could account for the difference.

Run a speed test from each of the computers.

Let's move this to the Apple forum.