If you do a lot of brushing and/or spot healing, the Lightroom Develop Module (and sometimes the Library module) is going to be slow. Turning on the Graphics acceleration in Lightroom CC/Lightroom 6 will make this problem worse. And I seriously doubt that Lightroom 5.7.1 will be much better if the brushing is the cause of your slowness.
You might want to try using Photoshop or Photoshop Elements for your brushing and spot healing. This will result in the rest of the tasks in Lightroom speeding up considerably.
If you still think you want to brush in Lightroom, do not do any lens corrections until you have finished brushing.
It would also help to get the fastest CPU you can afford ... and by the way is that a typo, your memory runs at 666MHz? That's kind of slow by today's standards.