Domain doesn't affect any speed of your website, Those people who talk about NameCheap being faster is difference in the Domain control panel which doesn't have any connection with that.
I need a more technical explanation than that. I hope someone could explain it to me in a more webby way.
Your server affects speed...if your on a shared server it could be the reason..if you have too many probs get a dedi.
Domain has no link with your Server Uptime or Speed Term Domain Name (Wiki) Registrar (Wiki) Hope that will you understand in more Technical way.
I guess it's possible that one of them would have faster resolvers (assuming you stick with their nameservers), but would still be very surprised if it made a noticeable difference!
If you set the nameservers for the domain to point to those of your hosting company then it is your hosting companies nameservers that controls the speed at which a domain name is resolved, the registrar is completely out of the loop. Once the domain is resolved to an IP then it is the speed of the hosting companies servers that you site is on that control the speed of your site, ignoring geographical constraints. If you keep the domain using the registrars nameserver and their hosting that is when you need to look to see which is faster.