First try all of the steps (including Advanced Troubleshooting steps) in this Blizzard Support Article:
Performance Issues with Overwatch
Now if you are still looking to squeeze out the best possible performance out of the game. Check out my Maximizing Performance Guide for additional tips.
[Maximizing System Performance for Overwatch PC]
by WyomingMyst
Welcome to the Maximize Overwatch Performance guide! Here you will find various tips and tricks that help users get every possible frame per second, the lowest possible simulation rate, and minimize spikes of system lag during matches of Overwatch. This guide will go over recommended settings in Overwatch, settings outside of Overwatch, and other tricks you can use to improve performance. Please note, this guide is intended to i…