The DNS is like a Phonebook, when you type in "www.apple.com" the DNS tells your Browser what ip address this is on, if your DNS does not know "www.apple.com" it asks another DNS, if this DNS does not know it either it will ask another DNS.....
So changing DNS will only improve initial load times , the Time until you see anything , not the time a page needs to fully load!
Also if just Safari is slow... try firefox and check if things get better!
ps: I always use OPEN DNS, for me its the fastest !
if things dont get faster, try to run a speedtest (speedtest.net ) and check what speeds you get there! if its slower than what you should have (check your ISP contact for that) try to use a cable instead of Wifi, if its still slow, check your router settings, it might show you the speed it runs on, if its too slow contact your ISP!