I have an older Macbook Air...a 2011. So I am leary that updating the OS to Yosemite will slow it down. Any thoughts or experiences here?
Don't even know if the update is worth the time and effort... The only thing I would be concerned about is the apple malware that's being talked about in the media and whether the latest build of Yosemite helps defend against it... Which I have no idea.
It'll be fine. You may have some graphical sluggishness due to the transparency effects; however, you can turn those off in the Accessibility menu in System Preferences. There's no reason not to update, IMO. There are two concurrent stories. One is about a site in China affecting Chinese users, the malware of which has already been blocked. There's a second story about a privilege escalation vulnerability that has been reported to Apple and won't be made public until a patch is released (presumably coming in 10.10.1).
You should be fine. I upgraded an older, lower model MacBook from 10.6.8 for a friend without issue. Maybe run CCleaner before you upgrade.