opmanager 10 slow, not usable

Hi all,
we've just installed OpManager 10, last release.
Now we're monitoring about 180 devices.
All of the sudden, the web interface has become very slow: about 10 minutes after service start, it becomes simply unusable (it takes about 20 minutes to login).
We've tried several things, we've disabled interactive service detection (both disabling win service and editing the opmanager xml file), we've tried to tune the postgres db (increasing the memory buffer and so on), but the issue still persists.
Task manager, when the interface becomes unresponsive, shows a too much little cpu usage (about 1-2%, nothing more), and a ram usage stuck at about 2.5gb (the server is equipped with 4gb).
Could someone please tell me how to proceed further? This thing is completely unusable this way.

thanks
bye