Data updated in CRM are not instantly changed on the Portal

Dear Experts,

I do use MSCRM Online with a Self Service Employee Portal for Cases management. 

The issue I face when a Case is updated in CRM is that the change takes a wild to appear in the Portal. 

I test it by changing the Case Title and after 25 minutes, the case Title was still not updated in the Portal - even if I log out/in. I had to manually restart the portal, then the updated Title finally appears correctly. 

It is not possible for end users to wait half an hour (or more, I didn't waited more) to see updated data in the Portal. 

I do have the impression that previously (a few weeks ago) the changes only takes a few seconds to appear…

On the opposite way, if a record is updated from the Portal, the changed directly appears in CRM.

How can we resolve that ?

Thank you in advance, 
Douglas