If Win7 you are screwed, just wait for Blizz to fix their code.
If Win10 here is a workaround how to get the game running normally;
Start menu-> Control Panel-> Hardware and Sound-> Sound-> Right click your playback device-> Properties-> Spatial Sound. Turn it ON.
Launch Overwatch, your fps is back to normal.
Here is the original post of the fix; Fix for FPS issue after patch-related to "spatial sound"
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