Not sure if its related, but I discovered (reported and accepted by the devs) that if you move too quickly to the World Map straight after the Main Menu appears, the FPS gets screwed up.
I wonder if this has some bearing on the general issue.
The workaround for the World Map FPS issue is not to jump to the World Map too soon after loading. Wait about 10 seconds to let the background processes settle.
Perhaps this screws things up and has adverse effects elsewhere. So far I have not had the issues that others report here.
Here is the related post.