Duolingo has the most incredibly inefficient web UI ever

I love how Duolingo makes learning a new language fun, and for free! But i have to say that the web UI is horribly inefficient. All it has to do, most of the time, is present you with a simple sentence which you have to translate. That is it... Looking at the HTML, it is pretty basic as you would expect, but after noticing my laptop's fan struggling like hell to keep my CPU's temperature down, I had to do a little performance test using FireFox... and whoah! this thing is running a million lines of React crap per second! It filled the profiler's buffer in a matter of seconds!!!

Can someone explain what's going on. All I can see is a lot of React stuff (e.g. reactVendor:react-redux/thunk...), but I don't really understand what any of it is doing... Why does it seem to keep doing stuff even when I am not really typing or anything??

It's so bad it's uncomfortable to keep typing for hours, as the keyboard is too hot!

Here's a link to Duolingo for those who don't know it: https://www.duolingo.com/learn

I recommend it for learning a language, but definitely not for learning how to design a lean User Interface :)