Re: open DNS
As NaMaan explained DNS resolves host names to IP addresses. So if you point your browser to "www.yahoo.com" it first gets the IP address from your designated DNS server and then the browser goes to that site. So the faster the "translation" is the faster the browser gets on with its job.
Your ISP may be bogged down on the DNS side or maybe they have older servers. Most DNS servers dont have that much load on so people tend to use older machines that do not have much resources. The other one that you started using may either not be very busy or they have faster machines.
One of the better ideas is to use one of those "nanny" DNS services out there that filter out bad, infected or dirty sites.