Hi Riccardo,
How big is this cluster you are monitoring?
I am not sure what the problem is but i can tell that the guys in the shop were adding many features and fixes related to scalability and the ability of the GUI with handling large fabrics. Please consider uplifting the UFM to the latest (currently 4.6.1) which will require you to also update your OS to > RHEL6.3.
it could be that your Java client is also causing this. clean your cash; make sure you work with Oracle Java webstart *latest revision* (other Java provider may not support all functions that UFM is asking for)