I have Lightroom 5.6. Whenever I use the Spot removal, Graduated filter, Radial filter or Adjustment brush tools in the develop module, it ties up the 6 CPU cores (as viewed in Task Manager) so everything goes very slowly. For example, the effect of a stroke with the Adjustment brush only appears on screen after 5 seconds or more. Sometimes it gets so bad that I have to kill the process and restart Lightroom. It affects other programs running at the same time, as they can't get enough CPU. It is basically unusable. An edit that should take five minutes can take an hour.
The problem happens when I either
It is not just an issue with Lr 5.6 - previous versions also did this on my system.
My workflow is:
I occasionally use Perfect Photo Suite 8.x on my images from within Lightroom.
I have been a software developer for over 30 years, so I'm not doing anything stupid. Lightroom is just spinning the CPU cores to no effect, like it has a thread or threads in an infinite loop.
I typically have Lightroom, Photoshop (CC or CS6) and InDesign CS6 running at the same time. All three together occupy just over half of the available RAM.
I have a prettty fast machine (see the specs below) and other software does not tie it up like this (except rendering video in Pr or Ae, which I expect).
I have tried googling for an answer, with no success.
I feel this is an issue that Adobe ought to provide technical assistance for, given the technical nature and severity of the problem.