Unity Editor hangs for a while when getting focus

Unity 5.6.3p4

Whenever I switch focus back to the editor, it freezes for about 10~20 seconds, and also does that whenever I open any menu (be it in the menu bar or context menu) for the first time after switching focus back to Unity.

Windows task manager shows no almost no processor use during the hang, but shows some disk activity.

I have already tried to delete the Library and Temp folders.

Any suggestion might help. As few as 10 seconds seems a small time, but this happens every single alt-tab and also subsequent menu-click. I must be losing hours of work in the last month and this is starting to get into my nerves.