Internet traffic slowdowns are a bit like road traffic problems: sometimes it's not actually a local problem, e.g. a problem on a motorway (the major net links) causing traffic to back up in to a town (your ISP). DNS problems are definitely the ISP, it's standard for them to run their own DNS servers.
But if it is a local problem, you can tell how good the ISP is by how well they spot the problem, identify the cause, and do something about it. My ISP here (Eircom ADSL) has had DNS problems for
years now - making them average by Irish service standards - but a quick web search found details of alternative (non-Eircom) DNS servers to use, which sped things up considerably.
