AFAIK SQL Server Express is limited to a 10GB database (plus other limitations) - it could be quicker or slower, depending on many factors. Adobe required also something that could work on macOS, and even Android/iOS.
Yet I would like to see LR supporting an external database and allow a catalog to be used by concurrent users - even if I'm afraid if it even happens, it will happen now in the cloud.
You can get a hell of a lot of file data (which is tiny) stored in 10GB, say you allocated 10,000 bytes per file (far too much) then you can manage 100,000 files in around 1GB (I'm not going to work out the GiB values )
In reality I am talking about a fully normalised database (I've built it already) which reduces the space requirement by a huge amount, a couple of thousand bytes should be able to accommodate a description of any file probably including the key EXIF data I want to record plus tagging and descriptions and so on.
I'm only building this for myself, if it proves useful to others that will be a bonus.