Have you tried updating to the latest? The latest update is 6.10.1.
What operating system are you using?
Have you tried turning off the GPU option in the Lightroom preferences?
How much RAM do you have installed in your computer?
This sounds like something I could try. Do you have any link I can use to update to the latest (not CC)? The application keeps showing I'm up to date...
I use OS X Yosemite and have 2x 2Go on my computer. Also, the GPU option is off... this is getting really frustrating and time consuming.
Here is the direct link where you can download the latest update. The one update will handle both Lightroom 6 and Lightroom CC 2015. There isn't a separate update for Lightroom 6 because it is the same program, just activated differently. You will find the update under the heading Adobe Photoshop Lightroom 6. You only need to download the topmost update which will be the most recent.