Mac mail slow due to 65gb of email?
Mac Mail saves the emails on the hard drive. I would try to find A mail app that doesnt save the mail to the hard drive but just sees the mail from the cloud email server. 

If you use Gmail as a mail client for those emails then I recommend Mailplane. Others like Spark, CanaryMail, Mailspring and Polymail are all good alternatives along with others in the Mac App Store.

Other email clients should offer an option to store less emails offline and limit the size of your cache to a manageable size.

While there are steps to clean out the space for Mac Mail, I personally think signing out of it altogether and using an alternative might be best.