A step-by-step guide to forcing Lightroom to rebuild preview files. This can help reduce the overall size of the Previews folder/database, which often retains outdated previews for files you've deleted, and therefore speed up the catalog as a whole. Below are some of the key steps:

3. Find all missing photos. Before you remove the previews, you’ll want to know if any files can’t be found by the catalog. You’ll want to do this while you still have the previews intact, so you can see what the missing pictures look like. Make sure all drives are connected to your computer or mounted over a network. Go to Library>Find All Missing Photos. You’ll get a notice from Lightroom that all photos are present, or that some are missing.

5. Close Lightroom and remove the Previews. The previews are contained in the catalog folder. They are named [CatalogName] Previews.lrdata. On Windows this appears as a folder, on Mac it looks like a single file. You can either move it out of the catalog folder if you have enough space on the drive, or delete it.

6. Rebuild Previews. Open Lightroom, select images, and go to Library > Previews > Build Standard Size previews. They will start to rebuild.

Tip: If you want this to go as fast as possible, it can be best to run multiple batches at the same time. Select half your files, and start the rebuild, then select the other half and start. This will force Lightroom to use as much of the computer’s processor as possible. It may cut the preview building time significantly.