Gmail Performance Issues due to Inbox Structure - Mac Mail App

The user has a gmail account that they are connecting to on a MacBook Pro 13 with High Sierra OS.
They are using the Apple Mail App

They have noticed delays when sending email and reading attachments.

I have noticed  issues with their gmail setup:

Do they have too many messages in their inbox? They have more than 18,000 messages in the Inbox.  I can have them fix this by creating folders, moving mail, and using the excellent MailHub Mail plugin to rapidly delete and file mail.

They have created a lot of sub-folders to their gmail inbox.  Are these folders slowing down the Apple Mail? Do I need to relocate them?
gmail inbox subfolders
I also noticed, that when I moved old email to a separate folder, there was a lot of download activity.  What is the cause of that?  I only moved a few hundred, but it looks like several thousand are being categorized as 'downloading'


Archive mailbox with activityDownloading Activity message
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Eoin OSullivanConsultantCommented:
I'd definitely recommend MailHub as an excellent plugin for Apple Mail for rapid filing of emails

I've several large GMAIL accounts setup in Apple Mail and that "Downloading Messages" xxx new messages is very common and not really an indication of anything wrong or an indication of the number of messages you're moving.  What is happening is GMAIL is simply syncing data over IMAP and re-organising emails, indexes etc.

The bad news is that creating subfolders in the Inbox on Gmail (or any IMAP) system will not improve performance as the IMAP sync system will still have to run through all the folders which are synchronised after every change/move or new email arrives.

The only way to improve performance is to move emails out of GMail completely and store them in a local Mail Folder which Apple Mail can manage on the local HD almost instantly.
Alternatively there are SETTINGS on Gmail to not sync certain folders over IMAP .. you could turn off the SYNC on a number of the subfolders to see does it improve performance.

Finally, I've also heard that if you ARCHIVE emails in GMail .. it reduces the IMAP sync loading (but personally I cannot confirm as I move emails to my local PC Mail folders once they are 6 months old and file them using MailHub myself)

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