Outlook 2011 (4.3.9) slow sync on Mavericks 10.9.1

My Outlook 2011 for Mac is super slow in syncing emails. I have 2000 items in my inbox and few sub-folders (not part of the Inbox folder) containing 1000 items each. When I send an email it gets sent instantly and when I receive emails that match rules, they get quick as well. It is only un-categorized emails that go to main Inbox are really delayed by 30min and more.

I can see constant messages at the bottom like: "Updating local folder" and "Determining changes"

Let me clarify few things before anyone suggests a fix:

1. It's not a network issue. Other devices on same network work fine.

2. I tried creating and new Identity and reducing number of items in inbox and sub-folders

3. I have late 2013 rMBP with 2.8ghz i7 with 16GB ram and pcie SSD so hardware is not a problem here.

What could be the problem? It's really annoying as I simply cannot use it for work.

Thanks

Dominik