Demon's souls running better when it is at the background?

I am trying to emulate Demon's souls (Vulkan) and it runs perfectly as long as I keep it in windowed mode with something else in front of rpcs3 window. When rpcs3 is running in full-screen mode or if I click the window, a lot of stuttering and fps drops happen.
I have never seen anything like that before, someone know if it can be fixed? I tried enabling V-sync ( from Nvidia control panel and rpcs3) and it does not solve the problem.