I posted this earlier but don't see it in the forum:
My setup is LR 6.4 on late 2012 iMac w/32gb RAM, OS 10.10.5. GPU is disabled in preferences.
My catalog is about 2.7 gb with about 65,000 images. It is optimized every day. Until yesterday everything was working speedily. Last night, working on some dngs of ~40mb from a Leica camera I began getting the beachball for a second or two every time I made a change to a radial adjustment. I created a new catalog and brought only one image into it and there was no beachball, the Develop Module was responsive.
I've read that people are working with much larger catalogs than mine without problems. Does anyone have any thoughts on the sudden change in Dedvelop Module behavior or suggestions on how to remedy it?
Where is your catalog located on your Computer?
If on an external drive try moving it to a new location and see if that helps.
Also have you tried rebuilding Previews and purging Cache? Refer How to improve and speed up Photoshop Lightroom performance
Previews are not used in the Develop Module, so it won't do you any good to delete and rebuild them, which in your case would take nearly forever.