Overwatch is a heavily threaded game with fairly complex rendering features. It will use as much of your computer’s CPU and GPU resources as you allow. If your CPU or GPU is not sufficiently cooled to handle running at the options selected in the game (along with any other applications that are running), the CPU/GPU might downgrade its performance while it is overheated. This problem can be exacerbated by overclocking of the CPU/GPU. In the worst cases it can cause your PC to automatically shut down.
So if you find yourself in this situation what can you do?
The first recommendation is to ensure that you have cleaned out the interior of your computer and all exhaust fans and heat sinks. This will allow your system to cool more efficiently.
Secondly you can try shutting down any other applications that are running. These applications can contribute to overall CPU/GPU utilization and introducing overheating.
If that does not address your concerns you should ensure that you have enabled LIMIT FPS and VSYNC in the video options. If you set both of these options to OFF you are instructing Overwatch to use as much of the CPU/GPU as it can. This will almost always result in either 100% utilization of the CPU (if your system is CPU limited) or 100% utilization of the GPU (if GPU limited).
Setting LIMIT FPS to DISPLAY-BASED will limit the FPS to slightly above the refresh rate of your monitor (60 FPS for most, though some monitors refresh at higher rates). Setting VSYNC to ON will ensure you see no tearing when the screen is refreshed. In some cases, such as older laptops or systems generally prone to running hot, setting the LIMIT FPS to 30FPS might be advisable.
If the GPU and CPU in your system are capable of preparing frames faster than the refresh rate then they will have an opportunity to idle during a portion of the frame (and cool down). This should resolve any concern with heavy utilization and/or overheating.