How do I go back to the faster Lightroom 5 now my files have been converted to LR6? Not only is my Macbook Pro GPU not supported, but the whole product is so slow none of the sliders work in real time any longer. I either have to go back to LR5 or find another product. LR6 doesn't work.
Your Lightroom 5 catalog still exists.
Find your Lightroom 5 application, open it, and it should open your LR 5 catalog. If for some reason, it doesn't open your LR 5 catalog, use File->Open Catalog and point to your LR 5 catalog.