OSX suddenly hangs/beachballs all the time - possibly caused by windowserver or aperture?
According to the EtreCheck post, your harddrive is about to die. Do the following a.s.a.p.: 


Backup all the data you don't want to lose on a different storage medium
Create a Recovery USB drive using https://support.apple.com/kb/dl1433?locale=en_US
Replace the harddisk
Either reinstall using that USB drive you made, or use internet recovery if supported


There is a slight chance that it's not the harddisk, but possibly the SATA cable between the harddisk and the mainboard. This cable is cheap and I would recommend to replace it while you're at it.

For harddisk replacement, I would recommend a SSD instead of a traditional harddisk, it will make your Mac feel like new. While you're at it, upgrading the RAM to 8GB or 16GB will definitely help a lot performance-wise and make your Mac last a few more years.