It is in my opinion no way OK to abuse your users resources like this. They trust you to perform only the task that your website claims to do, and not a whole lot of different stuff.

This is exactly like people deploying bitcoin miners in their products (i.e. the esea.net scandal).

Additionally there is already an array of different distributed computer networks one can join with a private computer, why not improve on this type of solutions, rather than forcing it down the users throat in a very immoral way?