Performance Issues with Outlook When Delegating Large Mailboxes (Exchange Online)

Hello!

We're using Exchange Online, and most users access their mail through the Outlook (classic) desktop app.

A common scenario in our organization is that users need temporary access to a coworker’s mailbox when that person is out on PTO. We typically grant this via mailbox delegation (Read and Manage + Send As), and the mailbox remains a standard UserMailbox, not converted to a shared mailbox.

The issue:
When the delegate accesses the mailbox, Outlook often becomes sluggish or unresponsive, presumably due to syncing the delegated mailbox — these are usually large (30–40 GB). We've noticed major performance degradation during this sync process.

What I’ve Tried:

• Unchecked "Download Shared Folders" (Account Settings > Account Settings> Change > More Settings > Advanced)
• Adjusted Offline Cached Exchange Mode to sync fewer months

Unfortunately, these haven't resolved the issue.

My Questions:

1. Is there a way to delegate access without triggering a full sync of the mailbox data to Outlook?
   (Users are not open to switching to OWA as they rely heavily on Outlook for their primary mailbox.)
2. What’s the best practice for temporarily granting access to large mailboxes in Exchange Online without impacting Outlook performance?

Any insights or workarounds would be greatly appreciated!

Thanks in advance!

-Adam