Ch1cane is correct. The Blizzard Browser is used for the main menu. Our team is waiting on some new tech to improve the performance of this, since we have seen issues with the FPS and performance dropping when on the main menu. Here’s the forum thread for the menu issue.
if you have a dedicated graphics card, you may have luck with changing it to use the dedicated graphics instead. We’ve seen this help for the menu issue and it should take some load off of the CPU.