Slow screen update when opening custom menu in SAP BPC 5.1SP3

I have created a custom menu in SAP BPC 5.1SP3. When the end-users open the custom menu, the custom menu does not open in one go (like when you test the custom menu when building it), but you can actually see the application building up the menu line by line.

Anybody had this behaviour before and has a clue how to solve it ?