Do the timed tests grow more demanding?
I've been doing the timed tests for half a year now. And I cannot say for sure, but I get the feeling that the clock is much less forgiving now that I have reached the highest levels. With each timed session, it seems to be rewarding me less and less time for correct answers. Has anyone noticed the same? Or is this behaviour prescribed somewhere?
No I haven't noticed such things. I use timed practice since the time I entered duolingo. I have another account and I a man in level 13 in Spanish and also completed my whole tree. I use timed practice to improve my fluency. If I do all 20 correct in the given time, then I get my fluency updates. For example, if I don't do for a week, then my fluency degrades from 25% to 24%. If I do all 20 correctly using timed practice just 1 time, it updates my fluency to 25% from 24%.