I think they really use your computer to mine coin
Not sure if this is even possible, but definitely an entertaining idea.Long story short: HTML browsers weren't made for games. We use HTML5 canvas for drawing the graphics, and that's very CPU intensive on older machines. We could/should switch to WebGL, which is faster, but then there are compatibility problems with older machines.
In short: if you want fancy graphics, you pay for it.
When I open this site my computer freeze.
It is possible to mine bitcoin using a browser. Bitcoinplus.com has webminer I even tried it to my forum in 2012 but many visitors complain.