Since before a browser can download a website it needs to resolve the domain name, slow nameservers can be a problem. It doesn't matter whether you use the same provider for DNS and website though.
Your hosting provider most likely has a lot more impact on your website loading speed. Of course if your site is larger it will load slower but there can be are huge differences between providers.
But even if your provider is fast, if it's not including a CDN it can be still very slow to users far away from the physical location of the provider's servers. This is how response times looked for mail.live.com:
(take some time last week).
As you can see, even though the response times are good in New York, they are really bad in Singapore.