Feels like it needs some sort of feedback when you type too slowly to get the next word. Plus maybe making it a bit more obvious that you can select a target speed for the WPM.
This is fascinating when picking a liminal wpm. Single hand words are so much harder.
I can cruise along until I hit words like "exact"
EDIT: You could make a list of bigrams as people type and average the wpm for words with that bigram, then you could weight the randomly chosen words by the difficulty of the bigrams, to really help people focus on their biggest stumbling blocks.