I just noticed today that for lessons that have 1k or more words, each page turn is slower and slower over time, and it crashes the browser tab after some time.
I checked Chrome Dev Tools and see that app creates a lot of DOM Nodes that are never removed when browsing lesson, and it can use a lot of memory in JS heap size metric. For me, it was over 1.5k DOM Nodes and over 600MB of memory when the app crashed.