Some of my photo catalogues are slow to start, use etc. if they are like 150 000 photos in them, so I find a way to reduce the size.

Quickest solution is to find your catalogue file on disc and duplicate (and rename it), then open the duplicate and remove (but not delete) all files from the library (except for the most recent photos). Then go back to the original catalogue and delete the recent photos.

Or export your event as a new catalogue under File.

Alternatively, right click and Save Metadata for the photos you want to move to a new catalogue. This creates small XMPs next to the RAWs (not sure about JPGs). Create a new catalogue and import the photos. You keep keywords, star ratings, brush adjustments etc. You only lose virtual copies and Flag as Picked ratings.