The resource monitor fingered a process named "mdworker", and Google took me to a page explaining that the real culprit is spotlight, which is indexing the disk contents.
Since I needed all the CPU power of my laptop to compile the latest server, I asked Google again, and I found a page with a method to disable Spotlight permanently. The recommended method involves root access, changing attributes of a long list of files, and a reboot. (!!!)
I could not afford the luxury of a reboot at the moment (and the instructions looked too obscure for my taste), and therefore I used an alternative (and more Mac-ish) way. I opened the spotlight options in System Preferences, found the privacy section, and added the whole Macintosh HD to the list of places not to index.
Ten seconds later, the CPU activity was back to normal.
I have enabled again the spotlight indexing during the night, and all is well.
