Turn off the Graphics Processor from the Lightroom preferences.
Go to Lightroom Preferences from the Edit menu
Click on Performance tab
Uncheck the Graphics Processor
Let us know if that helps.
Sorry, should have said it's the same whether the graphics processor is on or not. The Graphics card is an AMD Radeon 5770 with 3 monitors running at 2560x1440. The card is on the latest Windows 10 drivers but having said that they aren't releasing new drivers for this card any more. Also, it's Windows 10, 64 bit and as far as I know fully patched.
I also started with a new catalog file. I can see that if I scroll through the thumbnails in Develop at the bottom the memory climbs quite quickly in task manager, even if I dont do any editing - tlakes it up to about 6.5GB..
I tried minimising Lightroom for about 5 seconds then restoring - that fixes it for a short while. At the same time I minimise it I watch the memory usage on Task Manager. Memory usage is quite high when LR is running slow and then when I minimise LR the memory usage drops after a few seconds. When I restore it runs OK for a while until the memory creeps back up again. Looks like some kind of a memory leakage issue. Quite repeatable for me..