Getting aggravated with a problem is normal, but being uncivil towards me for delivering information about the scope of support is unacceptable. I fully understand the problem and do have a lot of technical knowledge for resolving supported issues here. But keep in mind: I am a volunteer on the forum, not a Blizzard employee.
It is very likely this is being caused by other software on your computer, or with temperature issues. There is no currently known bug that drops fps by 70 when using alt-tab.
Blizzard creates their hardware specifications (minimum and recommended) in order to provide a guideline for hardware that can run the game. As long as you are achieving 60fps or higher (you mentioned that you’re getting 80fps minimum) with the recommended specs, they do not consider only 80fps within their scope of support. Here are some examples of the staff mentioning this:
I recommend coming to a conclusion on the above thoughts and opening a ticket with the staff: Blizzard Support - Contact Support. Make sure your diagnostic files are in *.txt format and you have filled out ALL fields to accomplish this without errors. Ad-blockers can also cause errors when submitting a ticket. Perhaps they’ll have other tips for you.