I have a similar spec to your's (except my CPU: I5 4690K @ 4.3Ghz), and similar issue... the HDD is the culprit, as reported by others users. What work for me: both shadows options set to flat and flat only, visibility distance set to medium, and try to not play too much with the "zoom" fonction, you will notice freeze when zooming in/out. And one last, the trees: distance 10000, not more. You can try this:
http://forums.eagle.ru/showpost.php?p=2495081&postcount=7
This mod remove the shadows of trees, useful.