The Pirate Bay users complaining of CPU usage spiking up to 100 percent while just viewing certain web pages found that there was a javascript based cryptominer embedded in the website code. The Pirate Bay says that it is running a 24-hour trial to assess the suitability of embedded mining scripts as replacements for advertising revenue.
https://www.notebookcheck.net/The-Pirate-Bay-is-caught-stealing-from-users-who-are-busy-stealing-content.249166.0.html