I can't say with any any real authority but I doubt the upgrade will introduce any noticeable slowness. I've got a 2012 Air which runs Yosemite quite happily. Can you actually upgrade to just Mavericks?
If it doesn't already have an SSD, get one in.
My 2011 iMacs are running El Capitan very successfully.
I did the same last year to my Mid 2009 15" MBP. Runs perfectly which stop me from buying a shiny new one!
Do the SSD really speed a machine up.. I've a iMac 2010 version 2.8 ghz with 8gb ram. Thinking of getting a SSD into it than buying a new Machine
I have a late 2010 air and it runs osx captain great. and its a core 2 duo 4gbs ram a 320m and a 120gb ssd