Have you try to disable the GPU support on the Performance tab under the Preferences?
Are you performing lots of brushing or spot healing on individual photos?
No spot healing and little brush work. About 4 or 5 grads on some of the images.
The previews and smart previews files account for 80gb of disk space. I did a big prune of the catalogue yesterday but there are still 95,000 files in it.
Number of files and amount of disk space taken by the previews should have no effect at all on speed.
Did you try to disable the GPU as suggested above?