Placing Setting Details for Best Effect-- The Danger of Frontloading Your Story with Description

One of my online students is writing a very good mystery. He has plotted it well, and he's working on developing the characters.
Last semester in the twelve weeks of my online class, I focused him on pacing. What is the best pacing for a mystery? What elements keep the momentum going, the tension high? What drops tension, and even distracts the reade…