late 2015 iMac is very slow
I bought this computer refurbished from Apple in early 2016 and used it for 4 years with no problems. Then, this computer ended up sitting in a box (climate-controlled) for about a year, and recently started using it again. I noticed it was very slow right away. I decided to completely erase the hard drive and restart from scratch but that did not help any. It literally sits for minutes trying to open chrome and I'm always seeing the rainbow circle constantly. I really don't want to buy a new computer just yet, but this is getting annoying.
Any help is appreciated!
iMac Line (2012 and Later)