All machines tend to slow with time as folk fill the drives or add more stuff. I have a 10 year old Compaq R3000 and it's as fast as it was new and the OS has never been reloaded and this is just one out of dozens at the office that don't show any sign of what you asked about.

The Macbook Air is written about very nicely by CNET so I won't duplicate that here.
Bob