People stealing computing cycles (and thus cost of energy, bandwidth) in order to mint crypto currency (bitcoin et al).
While the amount of energy stolen for you can be mere cents per day, in aggregate they can be stealing upwards of 100’s of dollars a day from people who visit their website.
Block Coinhive.com in your adblocker, firewall and/or hosts file.
Most of these clowns use it without any configuration parameters which means your CPU usage shoots up to 100%.
At least a friend tells me that’s so ;)