Factory Reset MacBook Air makes it better? I have MacBook Air (13-inch, Mid 2011) I did never factory reset it. 
It's getting sometimes slow and have too much heat, I've changed the fan but still got some heat and slow function till I force quit all apps. 

Do you think factory reset can make it better? I'm aware to do that cause I have a lot of files to be removed and some are critical to being moved of its location.
You can reinstall macOS without resetting 

https://support.apple.com/en-us/HT204904