Should I upgrade Mac OS 10.6.8 to El Capitan on Macbook Air 2010?
I'm posting this answer from a late 2010 11" MacBook Air running El Capitan, and I have no performance issues. It runs very well. But this machine has 4GB of RAM, not 2GB, and that might make a difference.

Remember that 10.6.8 has not received security updates for years now, and will be full of unpatched vulnerabilities. It is a risk to keep running it.