This started maybe an hour ago. I noticed one of the opera.exe processes was using 25% of the CPU (4 cores), so I closed probably "heavy" pages, but nothing changed. Opened Opera's task manager, and only thing using the CPU is the browser itself with 100%. So I close Opera and that single opera.exe process remains in memory, still consuming 25%.
Without having updated Opera, this is not happening anymore, so I guess it was related to some Opera server as mentioned above or maybe a hidden internal extension.