What browser are you using? If you are using Firebox, then go to Edit>Preferences>Privacy&Security and look for what kind of tracking protection you are getting, even the standard option will block cryptominers, however if you have it on custom you could have forgotten to select the option to block cryptominers, anyway most likely what you are experimenting is just websites that use too much your graphic card, I do not like those websites since they are very inefficient but many people like shiny graphics so there is nothing to do about it.