Have you tried to diseable the GPU in preferences?
Because if your GPU is to slow whatever it passed the LR test it can slow down your work!
On my system win 7, i7 16go and HD 6440, a simple zoom I'm the develop module takes three times with GPU on!
Just disable the GPU support using LR / Prefs / Performances / uncheck Use-Graphics-Processor.
On my 5-year-old computer I had to do this because it was slow and stuttery and would give me a black screen once-in-a-while.
On my 1-year-old computer GPU support makes things a little faster.
What is GPU?
GPU is the Graphics Processor which applications can also use.
It is found in Preferences under the Performance section.