If Win7 wait for Blizz to fix their code.
If Win10;
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. If the sounds bother you (they are really bad), you can turn Spatial’s off ingame. Your fps will stay normal.
However, you need to repeat this every time you launch Overwatch.
Original post of the fix; Fix for FPS issue after patch-related to "spatial sound" - #42 by zoloz469-2683