本当の解決策はこれでした。
https://support.google.com/mail/answer/78892?hl=en
From the Mail menu, click Preferences > Accounts > Mailbox Behaviors
Drafts:
Store draft messages on the server > leave uncheckedSent:
Store sent messages on the server > leave uncheckedJunk:
Store junk messages on the server > checked
Delete junk messages when > NeverTrash:
Move deleted messages to the Trash mailbox > leave unchecked
Store deleted messages on the server > leave unchecked
Mac OS X 10.10.3 YosemiteでApple Mail.appからGMailへのアクセスが非常に遅いときは、私の場合は以下で解決しました。
1. Quit Mail
2. Move ~/Library/Containers/com.apple.mail to desktop
3. restart Mac
4. restart Mail.app