Safari adblockers slow down browsing
I use and suggest uBlock for Safari.  

UPDATE 2:  Even better perhaps is a Safari port, by el1t, of uBlock Origin. It is based on the latest version and @Scott says it works well. Available for download here on github. 

(I wondered why https://safari-extensions.apple.com/?q=ublock turns up 0 results.  Still does, oddly. https://github.com/gorhill/uBlock/issues/1596 explains why for a while there was no uBlock Origin for Safari.)

UPDATE: Another option is to try Opera.  It now comes with a built-in ad-blocker which, it's said, allows it to work much faster.  Safari has also received some enhancements that allow it to do a limited amount of ad-blocking very quickly.

Evidence the uBlock extensions speed things up