I’m trying to create a photo book but so slow to download photos from iCloud - is there another way?

Are you creating the book in iPhoto or Photos? The signature of your question says "iPhoto", but the question is in the Photos forum.

It may be easier to work on your photo book, if you create it in a separate photos library, with only the photos you need in the book. This second library would be stored locally and not sync with iCloud. A second advantage would be, that you could archive the library on a backups drive, once you have finished the book. This way you would not accidentally delete or edit photos that are used in the book project, see: Create additional libraries

You may be able to speed the process - collect the photos for the book in an album and right click on it and select Download originals to this Mac - that is faster than downloading each one as you need it

LN