Slow syncronising

I am setting up my accounts collecting emails from Gmail and the syncing for one of the accounts is really slow. After 3 days of waiting only 400 of 5 000 emails are synced. And for the ones that are visible, It only shows the subject line - not the content.
The other account, also Gmail, is not problem. Synced all email in an hour and shows all content. Both accounts have the same settings. The only difference I can think of is that one of them has forwarded addresses - other addresses that auto forwarded to the account. Could that be the reason?

  • Are the settings the same at Gmail and in emClient?
  • Are there possibly huge attachments accompanying the e-mails of the slow account?
  • How many mails were synchronized in the fast account?

The forwarding should not be a reason for the delay - it would at least be surprising.

Thanks for quick feedback
The fast account synchronized some 10 000 emails
The largest attachments are no more than 25 MB
The settings are a bit difficult to verify as gmail does not specify ports, but I made some changes and testing at the moment.

25 MB attachments … – how many? This is – as I remember – a real “performance killer”. Which is obvious: It takes correspondingly long to download huge attachments.

And it makes a big difference if some or most emails have attachments. Is the average of mail attachments comparable between “fast” and “slow” account?