Outlook 2016 slow sync

Hi,

I'm using latest ZeXtras with Outlook 2016 and Zimbra 8.7.

ActiveSync runs correctly, but synchronization is very slow.

My account has 6 Gb of datas and many folders.

With Zimbra Outlook Connector (Network edition), synchronization takes about 4 - 6 hours.

With ZeXtras and ActiveSync, not all the emails are synced after 2 days. Outlook is configured to send/receive each 10 minutes.

How can I speed up this ?

Thanks

Regards,