It doesn't matter who you purchase your domain from, from a nameserver point of view. Once you register the domain, you should be using your own name servers and not the registrars. The speed comes in from your name server selection.
The one thing I would consider is how long the registrar has been around and how reliable their business model is. There have been stories in the past (RegisterFly) where the registrar disappears and folks started having problems registering, transfering and updating their domain names.
Fortunately the ICANN has a pretty good handle when it comes to Registrars. Should your registrar go out of business or have issues such as RegisterFly, they will take over the domain registrations and in this particular case, transferred all registrations to GoDaddy.
If you stick to an ICANN accredited registrar you are fairly protected from any type of issues like this. For this reason, I would stick with a registrar that is directly accredited by ICANN and not a "reseller". Some of the "top tier" registrars are GoDaddy, eNom, Tucows, Network Solutions, etc.