Ever since I upgraded to Windows 10 my entity model has been nearly impossible to update. It takes forever to open the model designer, then forever to right click the designer to bring up the context menu to select "Update model from database". Opening the same model in VS 2015 but on Windows 8.1 is much, much faster. It doesn't seem to be just my machine since the same is happening for our other developers on Windows 10. But those on Windows 8.1 have no problems.
I had exactly the same issue, and resolved it by doing the following:
Though you may not have to, I left "Enable rich client visual experience" unticked.
There's also another user who is having this issue, for whom the aforementioned fix does not help, it might be worth keeping an eye on their post, too, to see if anyone has any further ideas. I've included a link to their question below. :)