When installing free software be extra careful when you install it to make sure its only the software you have downloaded and not the bolt on (bundle) software included in the download. I recently had a macbook air in which was running very slow with just safari open. In activity monitor I noticed macbooster was running along with other apps which I could not identify.
By downloading adware medic (Malware Bytes) running that and cleaning out the adware including macbooster (2 reboots to completely clear the machine) the Macbook Air is not running very nicely with out the spinning beach ball of death. You can click here to see my other article on adware and anti virus software for mac.