If you are having issues with sluggish Outlook with a Microsoft Exchange account, try enabling Cached Exchange Mode.

Here's how to configure Outlook:

In Outlook:

File → Account Settings → Account Settings (drop list)

Then:

Email (tab) → select Exchange account → Change (button)

Further:

Exchange Server Settings (window) → select Use Cached Exchange Mode (checkbox) → Next

You should be back at the Email Accounts window. Click Finish.

You should see a notification window that advises you to restart Outlook in order for the change to take effect.

When you re-launch Outlook, it will begin to make a copy of your Mailbox contents onto your local computer. This will take some time. Not all items will be immediately available. If you have a large mailbox or a slow Internet connection, this can take an hour or more.

In Outlook, go to:

Tools → Account Settings → Email (tab) → Microsoft Exchange Server → Change

Then:

Exchange Server Settings (window) → select Use Cached Exchange Mode (checkbox) → Next

You should be back at the Email Accounts window. Click Finish.

You should see a notification window that advises you to restart Outlook in order for the change to take effect.

When you re-launch Outlook, it will begin to make a copy of your Mailbox contents onto your local computer. This will take some time. Not all items will be immediately available. If you have a large mailbox or a slow Internet connection, this can take an hour or more.