The mailbox used to be 11GB and we removed older emails so it's only about 3.6GB now.

Internet speed it good - cable modem at about 100Mbps down and 17Mbps up. I've set it not to update unread count and several other possible tweaks. I've turned off Starred, Snoozed and Important folders for IMAP. I've set Gmail to only allow synching of 1000 items in each folder. I've gone through selective folder synching and only synching/subscribing to the Inbox. This other computer has Outlook 2016 and not the newer 2019 v1906 that's on the new computer, but the old laptop of problematic user also had Outlook 2016 and had problems. While it's a little slow, it's much more responsive than the main user of the problematic email account. The weird thing is that another computer being used in the office has another 2 gmail accounts plus this problematic account and it seems to be able to send and fetch WHILE the sync is in progress. His old laptop did something similar with outgoing email taking a while to initiate sending or for new email to get fetched and they thought a new computer would fix it. Once the sync completes, outgoing mail sends and incoming mail comes in, but then it will sync again and no mail transfers until it's done. I've set the sync interval to 1 minute or 5 minutes or 30 minutes. I've tried unsubscribing to all the folders but the Inbox. I'm pretty familiar with the way Outlook and Gmail work and have had problems with it in the past as many people do (as evidenced by all the problem posts found on Google).

Sometimes it starts over and sometimes it goes up to 8 minutes. The problem is that in the status box, it shows synchronizing subscribed folders and usually shows about 2 minutes. I'm familiar with all the basics like turning on IMAP in Gmail (which was already on since he's been using the account already with his old computer and Outlook). I set up a new computer with Outlook 2016/2019 and connect to his existing account in IMAP mode. I have a client using regular email accounts.

Set up gmail in outlook 2016