When I take Outlook out of cache mode into live mode, sending emails take much longer when adding attachments/screenshots. This is not the in case in the Exchange 2007 mailboxes, only for Exchange 2013 mailboxes (currently in coexistence).

Any idea how to troubleshoot this issue?

You could start by using Performance Monitor to see if your server is experiencing any bottlenecks.

Which specific exchange parameters you would suggest to monitor on perfmon?

You can start with Processor > % Processor Time and Logical Disk Avg. sec/read (<20ms) and Avg. sec/write (< 10ms).

Hi,

During my research, I find an similar thread about your question, for your reference: https://social.technet.microsoft.com/Forums/exchange/en-US/a07b9269-be9f-4337-a394-9faf532d38f8/exchange-2013-online-mode-slow-performance?forum=exchangesvradmin

"1. Start Registry Editor (Regedit.exe).
2. Locate and then click the following registry subkey:
   HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Interfaces
   the interfaces will be listed underneath by automatically generated GUIDs like {064A622F-850B-4C97-96B3-0F0E99162E56} 
3. Click each of the interface GUIDs and perform the following steps:  
   a. On the Edit menu, point to New, and then click DWORD value. 
   b. Name the new value TcpAckFrequency, and assign it a value of 1. 
4. Exit the Registry Editor.
5. Restart Windows for this change to take effect. "