Sometimes sites can just be slow (overloaded webserver, bad downstream connection, network issues), rarely it is DNS related, DNS should resolve instantly and once a site has been opened once, the DNS is almost always cached in your system. So loading the same site in a row should already be enough to exclude a DNS related problem.
General slowdown when having issues in page loading I would suggest making sure that you do not have any unnecessary plugins in your browser, try opening an "in cognito / private " (this usually disables plugins) browser window and see if the speed improves.
If you initially had DNS problems I can recommend the google DNS servers over your provider's as a rule of thumb, I control numerous servers and remote computers, they all use google DNS as the default configuration and I've never had issues related to them (provider DNS is another story).
If you wish to dig deeper in the subject, you could open a cmd window (windows key + R , type cmd, press enter) and run an "nslookup" command, for example 'nslookup amazon.com' should tell you the IP adres behind the website and give you an idea how fast they get those results.
If you wish to determine why a website is running slowly you could run the (in chrome/firefox) network tab m in chrome this is accessed by pressing F12 and clicking the Network tab, then refreshing the page, this should reveal exactly what is slowing up the page loading and find potential issues.
example result of the network tab: https://developers.google.com/web/tools/chrome-devtools/network-performance/imgs/network-panel.png (this site displayed there loads completely in under a second, which I would say is a fairly good result)
All of this expecting that your own connection is good, be sure to test it on a computer that has wired connection, wifi in general is the cause for many reliability issues in general connection stability.
Sorry for the long message, hope it helps :)