Why this shared mailbox is so slow opened in ms outlook?

This is using MS Exchange server 2013. We have this shared mailbox in which quite a number of users are using. All these users configured to open this shared mailbox in their MS Outlook. Both user and shared mailbox are configured to use cached mode.

However, recently we found out that this shared mailbox is "sluggish" and slow. Sometimes, emails will only be seen after may be up to 30 minutes. Is this the norm for shared mailbox. Or else, what could be the best practice to use this shared mailbox?

Thanks,