ISP DNS vs OpenDNS

Have you tried to download the same files using both DNS? This would give you a measurable result that you could use as a comparison difference to support your decision on which to use! This would be based upon facts and not an opinion btw. Furthermore you personally will know that truth based upon your own research, and your findings would leave no room for doubt.

One of the most common tricks to speeding up your internet bandwidth is to unplug your modem & router (if you have one) leave them off for 10 minutes + and then turn them back on, what this does is it shakes off those who may be pinging your computer. If you don't have a router you may want to get one, these can increase your speed significantly if you are having issues, part of this is due to a very effective hardware firewall which can setup to improve bandwidth performance.

Cisco goes through a lot of research on how to improve bandwidth, so if you can afford Cisco, they have great products, and their routers often have less problems then other cheaper routers.

Remember to restart your computer after you have unplugged your modem.

Also, you may want to scan for Viruses, Malware & Spyware, these can slow you down significantly too. (This is actually the first step by the way.)

Assuming you're using Windows, try disabling the QoS Packet Scheduler in the Properties window of the your network adapter.