I just upgraded from Aperture to Lightroom 6 on my mac. I have around 6600 images (mostly RAW) in my catalog. I have the imaged located on a Firewire 800 external 7200 RPM drive. When I launch Lightroom it takes more than a minute to show my images in grid mode. What is going on. Aperture is so much faster. I've read about creating previews, but I'm confused on what kind of previews to generate. I am running on a new iMac 1920 x 1080 (21") display. Can someone please be specific on what I need to do to make this dog work like (I hope) it should? I'm pretty disappointed so far.