Downclocking your GPU would naturally mean a larger percentage would be used. If it needs say 200Mhz to do the rendering and you downclock to 250Mhz then it would say 80%, but when the GPU runs at 1Ghz it would show only 20%. No change in actual usage.
Blur is computationally expensive. You can opt into the less complex blur (Win7 style) from the WB settings window - See Force non acrylic blur on later Windows 10 versions.
You would probably find putting calculator on top of the video and giving it focus would also be costly GPU wise when you have downclocked your GPU as the calculator window would also show blur in the background. It may be a little less because it is one window vs multiple, but the principle is the same.