When you synchronize folders, you have the option of adding files that have been added to the folder but not imported into the catalog, removing files that have been deleted, and scanning for metadata updates. The photo files in the folder and all subfolders can be synchronized. You can determine which folders, subfolders, and files are imported.
- In the Folders panel, select the folder you want to synchronize.
- Choose Library > Synchronize Folder.
- In the Synchronize Folder dialog box, do any of the following:
- To import photos that appear in the folders but have not been imported in the catalog, select Import New Photos. If you select Show Import Dialog Before Importing, you can specify which folders and photos are imported.
- To remove photos that have been deleted from the folder but not from the catalog, select Remove Missing Photos From Catalog. If this option is dimmed, no files are missing. (You can choose Show Missing Photos to display the photos in Grid view.)
- To scan for any metadata changes made to the files in another application, choose Scan For Metadata Updates.
- Click Synchronize.
- If the Import Photos dialog box opens, specify the folders and files you want to import, and then click Import.
try to remove pictures from the catalog while syncing...
It may be faster to select the images in Lightroom, press Delete key, and pick the Remove option instead of the Delete from Disk option. That does no more than clear those selected images out of the Catalog, and mark the cached previews for deletion. Since these are pure database-style tasks they should (afaik) be simple and quick for LR to carry out.
The folder will still be showing in the LR Folders panel, now with 0 images inside it, but you can Remove that entry also (provided you do not have any images still imported which live in subfolders of that). Again this is a database task only.
Then it is up to you what you separately do with that physical folder and the physical files inside it, externally, using Finder - since LR will no longer know or care about them.