Do you use TCP Optimizer? If so, on the "Advanced Settings" tab, you should set the "Host Resolution Priority" at the Windows default. You can click on the "Windows default" radio button on the bottom of the TCP Optimizer screen to see what they are.
A couple of years ago I learned the hard way that allowing TCP Optimizer to set these values at the "Optimal" setting causes a conflict with the "Real Site" component of Avast and prevented DNS resolution. My educated guess is that having such high priorities (low numbers) for the priorities caused a race condition with the "Real Site" component, so most DNS requests ended up not being processed or processed at a snail's pace.
At that time I jotted down the default settings, then chose optimal and set them, rebooted, then set custom and changed the "Host Resolution Priority" numbers back to the Windows default, saved and rebooted, and that seemed to work fine with Avast's "Safe DNS", which is now called "Real Site".
