Exchange 2013 CU6, 500 users, Online-mode

Recently we are facing the outlook performance issue, outlook freeze, slow response.

Considering the security, it's online-mode used. However online-mode gives bad performance on Clients and Server.

Would you please tell me about the online mode, how the client connected by online mode

is effected by Network and Server issue? including the number of Items and calender size.

You are welcome to download the Exchange 2013 Server Role Requirements Calculator and see if your server has enough capacity.  However, I think you've learned the painful way that it does not.  Someone here might be able to offer better advice if you'd shared the hardware (or virtual) configuration of the server.

https://gallery.technet.microsoft.com/office/Exchange-2013-Server-Role-f8a61780