Custom (competitive) Timed Practice with High Score
I see many people complaining about the timed practice, but personally I like it.
A good improvement however, is to have a custom timed practice. The general idea is that you set a time maybe 5 minutes or 10 mins or so, and attempt to answer as many questions as possible.
Another idea would be to specify the number of questions to answer and let duolingo decide how much time is required.
There could also be personal/friends high scores that you may want to beat, for preset times such as 5/10/15 mins for specific skills.
I generally like to practice a skill(block) until i can remember it quickly, 30 seconds with 20 questions is not enough for me to master/memorize it and I tend to click "start practice" many times which can be tiresome at times.