This article contains general tips and links to other resources that tell you how to improve performance of Exchange Online.
This article is part of the Network planning and performance tuning for Office 365 project.
Things to consider in order to improve Exchange Online performance
To improve the speed of migration and reduce your organization’s bandwidth constraints for Exchange Online, consider the following:
Reduce mailbox sizes. Smaller mailbox size improves migration speed.
Use the mailbox move capabilities with an Exchange hybrid deployment. With an Exchange hybrid deployment, offline mail (in the form of .OST files) does not require re-download when migrating to Exchange Online. This significantly reduces your download bandwidth requirements.
Schedule mailbox moves to occur during periods of low Internet traffic and low on-premises Exchange use. When scheduling moves, migration requests are submitted to the mailbox replication proxy and may not take place immediately.
Use lean popouts for Outlook on the web. Lean popouts provide smaller, less memory-intensive versions of certain email messages in Microsoft Edge or Internet Explorer by rendering some components on the server. For more information, see Use lean popouts to reduce memory used when reading mail messages.
For more information about Exchange migration performance, see Office 365 migration performance and best practices.