Have you rebooted your router? (this may seem obvious but some people forget to do this)
Have you contacted your ISP to see if there are network issues? (You may have the correct download speed but this doesn't reflect latency issues)
If you believe its a DNS issue have you tried changing your primary dns server to another? (You can use googles public dns server 188.8.131.52)
Do you update your hosts file? If this gets too big it can cause network issues
Have you tried only having one computer on the network at a time to see if one of the machines is flooding the network?
How are you connecting? Wireless or wired? If it wireless try using a wired connection and see if improves things