A DNS resolver basically translates human friendly web URLs to IP address. Then the browser connects to that IP to fetch the web page. This happens every time you connect to a website, send an e-mail, or do almost anything on the Internet. You can estimate how critical this part is, in terms of your browsing speed.
By default we use the DNS server of our ISP. But there are some good alternatives including Google DNS or OpenDNS. All these have nearly same speed but it depends on your location and connection you are using. There is nice tool to check which one is best for you.
Go the DNS Benchmark page, and download the utility. Its just 183 KB in size. After that, open the file.
Go to Name servers tab. It will take few seconds to initialise all NS. When it finishes, click on Run Benchmark button. Now it has started the analysis. It may take a couple of minutes depending on your speed.
When it finishes, you would have a sorted list of all major DNS servers according to their response time. Here is the test result for me. And my ISP (NIB-BSNL Broadband) is at the top. Google Public DNS is at second.
I am not using any 3rd party DNS server, and should not, as the results says. But, a long ago, when I was using Tata Docomo 2G Internet, the tool ranked the native server at 2nd, and Google at 1st.
Anyway, the scene may be different for you. Do check it, and share in comments.