What driver are you using? Did windows install a new driver for you?
I was using an older one that worked fine, I tried installing the most recent and it didn't change anything in performance at all. It's a Windows 7 64 bit, I have updated it to a good point but I have automatic update disabled. So in like 7 months it didn't update at allTake a test with this driver:
https://support.amd.com/en-us/kb-articles/Pages/Radeon-Software-Crimson-ReLive-Edition-Beta-for-Blockchain-Compute-Release-Notes.aspx