This is a new feature that I had ignored - My Bad!! One of the critiques of Lightroom has been that on Import, it is much slower than PhotoMechanic in rendering the Previews - the photo you see in the LR window. PhotoMechanic is lightning fast, as soon as it opens a photo, it shows up fully focused immediately, while LR takes a second or two. This can make a big difference when culling, screening, ranking, and adding Keywords to a big import - like thousands of event photos! I didn't realize this, but PM reads the Embedded Previews (jpeg) created by the camera itself. These jpegs are what you see when you look at the camera's LCD. Up until December 2019, LR didn't have this capability. LR creates its own Preview and ignored the cameras Embedded Preview - hence that bit of time for that photo to show up sharp on your screen.
But! As of December, LR added the option to use the Embedded Preview when importing. Wow - for those large shoots, this is Awesome! We can now see the full sized photo in the Library immediately. This speeds up Culling, Ranking, adding a Color, and Keywording a LOT!
The Embedded Preview will carry the camera's Picture Style: Landscape, Vivid, Black and White, etc; so it will look like the image on your camera's LCD. Once you add a change in Develop, the Embedded Preview is overwritten with a new LR Preview showing the changes you have made. Note that Importing with a Develop Preset will overwrite the Embedded Preview, create a new LR Preview & you will lose that speed advantage. So for large imports, I no longer import with a Preset, I'll apply that after I finish my ranking and keywording.
This is a very cool feature & I'm already loving it!!
Thanks to David Coleman Photography & his Have Camera Will Travel website for his excellent explanation! https://havecamerawilltravel.com/lightroom/lightroom-classic-new-embedded-previews/