Have there been studies about the effect of idle time vs time pressure in learning?
There is an article on college students under pressure. It details on the effects and the health related issues involved.


  Amy Novotney, "Students under pressure," American Psychological Association, Vol 45, No. 8, pp. 36 September 2014.


Although it doesn't explicitly account on idle time, you should be able to draw conclusions from the content provided.