Lightroom uses previews to display photo thumbnails in the Grid view, the Loupe view, and in the Develop, Slideshow, Print, and Web modules.
When you import photos, you can choose from three types of previews of progressively higher quality:
Minimal: These previews are the small, low-resolution JPEG previews embedded in the photos, which the camera generates. They are the fastest type of preview to create. The Filmstrip and Grid view of the Library module uses minimal previews temporarily, until Lightroom renders standard-size previews for those thumbnails.
Embedded & Sidecar: These previews are larger, also camera-generated, and they take a little longer to create than minimal previews.
Standard: Lightroom creates standard previews. They use the Camera Raw engine for processing. So, the sometimes appear different from minimal or embedded previews, especially if you have applied adjustments in the Develop module. You can specify the size of the Standard preview you need, based on the display you use. Standard previews are used in Filmstrip and Grid view thumbnails, as well as in preview and content areas of the Slideshow, Print, and Web modules.
1:1: These previews are a 100% view of actual pixels and, like Standard previews, the Camera Raw engine processes them. When Lightroom generates 1:1 previews, it also generates minimal and standard previews, so all three are available to the program as needed. Because so much data is being processed, 1:1 previews can take a significant amount of time to create. Any time you zoom to 1:1 or higher in the Library module, Lightroom uses 1:1 previews.
To display and work with photos, Lightroom requires a standard or 1:1 preview, depending on the task. If, upon import, you only tell Lightroom to generate Minimal or Embedded previews, Lightroom creates Standard and 1:1 previews automatically as you’re working in the application. This process hinders performance. To increase your productivity and reduce this disruption, manage when and how you render your 1:1 previews. Render them on import, or set aside time to render them manually.
To render 1:1 previews on import, use the File Handling panel of the import window. Choose Render Previews > 1:1. (See Specify initial previews when importing in Lightroom Community Help.) Although generating high-quality, 1:1 previews on import slows the import process, it makes Lightroom more responsive when you start to work in the Library module.
An alternative, if you want a speedier import process, is to render minimal or standard previews on import. Then, at any time, select multiple photos in the Grid view of the Library module and choose Library > Previews > Render 1:1 Previews. Let Lightroom process the images before you start to work on them.