Categorizing mails is slow in Outlook when cached mode is disabled

Hello,

I'm in the middle of the process of migrating Exchange 2010 to Exchange 2013 for my company, and I'm encountering the following problem:

If a user migrated on Exchange 2013 is categorizing several mails in Outlook using "Quick click" when cached mode is disabled, there is a progression popup that appears saying "categorizing items...", and it last 3-4 seconds. What I mean by several mails is at least 10 mails, which is not "big" I think.

Ok, this is no big deal, but this was not the case on Exchange 2010, and my uses are starting to complain. 

Enabling cached mode solves the problem, but we cannot enable it for everybody.

Outlook is up2date.

Any ideas ? Is this a normal behavior ? 

Thank you

MK