Force Outlook to download Cached Mode data at full line speed

I have a number of users on Office 365 with large mailboxes (10GB+) and whenever I need to recreate the cache (cache or profile corruption, OS reinstallation), Outlook rebuilds the offline cache but at speeds consistently around 1Mbps - internet connection is 4Mbps. Is this throttling on the Office 365 server side, or is this client side? Can anything be done to force a faster initial population of the cache to reduce the time to get up and running?

Thanks

Patrick