New Outlook on Mac is slow to open and respond to emails, takes from 5 to seconds for emails to load or reply but Legacy Outlook works fine.

The issue you are experiencing with the New Outlook on Mac being slow to open and respond to emails could be related to several factors. Here are some potential solutions and considerations:

1. Antivirus Software: If your antivirus software integrates with Outlook, it may cause performance issues. Consider disabling any Outlook integration within the antivirus software or any add-ins that are installed in Outlook.
2. Large Data Files: If you are using a large .pst or .ost file, this can lead to application pauses. Files larger than 10 GB may cause noticeable delays, especially when performing typical operations like reading or replying to emails.
3. Profile Issues: Since you've recreated the Outlook profile multiple times, ensure that the new profile is set up correctly. Sometimes, corrupt profiles can lead to performance issues.
4. Cached Exchange Mode: If you are using Cached Exchange Mode, consider adjusting the settings related to the synchronization of calendar items or other data, as this can impact performance.
5. System Resources: Ensure that the new MacBook has sufficient resources (RAM, CPU) to handle the New Outlook application efficiently.
6. Updates: Make sure that both macOS and Outlook are updated to the latest versions, as updates can resolve performance issues.

If these steps do not resolve the issue, you may want to consider reaching out to Microsoft support for further assistance, as it seems to be affecting multiple computers.

References:

• Outlook not responding error or Outlook freezes when you open a file or send mail
• You may experience application pauses if you have a large Outlook data file

Hi,

The delays in New Outlook for Mac 10+ seconds to open or reply are not caused by local cache or add-ins they’re due to how the new Outlook architecture works. It’s cloud-first, so every action depends on Microsoft 365 services. If your mailbox is large, has many shared folders/calendars, or your tenant/network is under load, Outlook waits for server responses. Microsoft also applies throttling during heavy operations, which can slow things down without showing errors.

What I suggest is:

Update to the latest Outlook build.

Reduce complexity: turn off Conversation view, temporarily remove large shared mailboxes/calendars.

Reset Outlook preferences and test on a clean network no VPN/proxy.

Check Microsoft 365 Service Health for advisories or throttling.

Keep Legacy Outlook as fallback until Microsoft resolves performance issues for New Outlook on Mac.

Let me know how it goes, Happy to help!