Mail app really slow to move messages, open the content, etc

Good afternoon all,

I've looked around in the community about this problem, but I couldn't find a solution for it. Maybe today explaining better the situation, I can find someone who found a solution.

I have 5 iMacs, with High Sierra. All computers started slowing down the mail app. The first thing I tried was, erasing the hard drive and installed fresh installation of High Sierra. When I finished the installation, I went straight to the mail app and added the email account. Started downloading messages. It's been 3 days downloading messages, and it seems to be downloading in loop. Well, the mail app is slowwww.

I have tried rebuilding, but no success so far.

All computers use the same email account. All computers I receive and send the same emails.

It is a @gmail account.

I thing I gave all information basically. If anyone knows anything about this, please let me know.

Thank you