Ok fixed it.

First I realized that it only happens when I'm online.

Then I saw a high memory usage of osso_rss_reader in load-applet's process list and I killed it forced twice and voila the cpu "cooled" down.

Note that I was not having the RSS Reader home applet activated.

I tried to launch the RSS Reader from the Applications menu, the reader didn't come up, just a white window with the title "RSS Reader" loading forever.

Finally I just deleted the folder ~/.osso_rss_reader/ with terminal and since then everything is OK again.

Be careful if you do the same, deleting that folder means that you delete your settings and lose your feeds. But I don't use the RSS Reader anyways.