^ Do you think its os sierra or safari 10 causing it (the beachballs)? cause you can put safari 10 on el cap, in an update over app store
I partitioned 40gb of my 256 on 13" for El Cap, to compare between OS'es. On 10.11.6 with it, and 9.0.1 safari,
and recently loaded win10 pro boot camp on my 2015 12" rMB 40gb cut up, with MacOS sierra for the rest too,
Too bad I forgot that once you go to a certain OS version with certain firmware on rMBP, can't load Mavericks (wanted to try now old school skeumorphic on a retina screen again, such a nerd)
My late 2013 came out when Mavericks was still the latest and greatest, but unable to load it now.