https://support.google.com/a/answer/1071518?hl=en

Part of supporting an open communications standard is making it an equal citizen on your network.  With Gmail, you can use the IMAP protocol to connect your Gmail account with an email app of your choice.  By using IMAP, Google maintains its Gmail service is compatible with 3rd party apps.  It’s what prevents you from being trapped in a web browser and abandoning the email or collaboration clients you’re used to.

The trouble is that even though Google support the IMAP protocol for Gmail, it throttles your connection to save on bandwidth.  If you have a large Gmail account and you set it up for the first time on your PC or Mac, be prepared to wait hours or even days to sync all of your email.

Google throttles IMAP connections to both save on bandwidth and make it difficult to leave their service.  After years of IT consulting, we have experienced the pitfalls of Google’s throttling.  It usually becomes a problem at the most inopportune times.  Like having to rely on a temporary computer while yours is being serviced…  Now imagine you’re on the road without constant access to the Internet.  Because Google severely restricts the bandwidth for IMAP, it’s unlikely you will ever download all of your mail in time for it to be of use.  You just won’t have access to your email unless you resign yourself to a web browser.

Supporting open standards and making them equal citizens helps customers avoid being held hostage by making their data easy to migrate.  Google goes out of its way to make the process of leaving them difficult.  We’ve seen clients take literally days to download their Gmail accounts.  By severely limiting the rate at which you can retrieve your data from Google Apps, Google aims to make the process difficult enough to back out or avoid it all together.  Knowing this, our team makes sure they take into account the time to actually sycn your Gmail before we migrate you.

We treat all communications standards - both open source and proprietary - equally.  We don’t throttle your data.  We expect you to download it, upload it, and sync it as fast as your Internet connection will permit.  Backstage ensures that no matter the scenario, we don’t get between you and your data.  We don’t hold anyone’s data hostage.  Part of believing in an open web is also believing customers should always have a choice to freely and easily switch providers.  It’s up to us to provide a service good enough to make you stay on its own merits.