I too would love to know the reason and solution for this. Same problem.
Lots of adjustment brushes and lots of spot removal can bog things down. Your best bet is to move those takes to the end of your develop cycle.
Had the same problem today with Lightroom 5.7.1. Every module I tried (Develop,. Book and Library & buttons) modules very slow. To make a long story short, I updated my AMD Radeon R9 370 driver and solved the problem.