https://discussions.apple.com/thread/6603972Safari running slow in Yosemite
From your Safari menu bar click Safari > Preferences then select the Privacy tab.
Click: Remove All Website Data
Then delete the cache.
Open a Finder window. From the Finder menu bar click Go > Go to Folder
Type or copy paste the following
~/Library/Caches/com.apple.Safari/Cache.db
Click Go then move the Cache.db file to the Trash.
Quit and relaunch Safari to test.