Save As copies the entire contents of the project so yes, it should take about the same amount of time as a backup.
Not directly relevant to this thread, but please be aware that the Save As command is a very common cause of “lost” work. It creates a new copy of a project, and then continues working in the copy. It’s very common for people to then open the original version and panic because work done in the copy was “lost.”
If you want to make a copy of the project independent of Scrivener’s automatic backups, I recommend using the File → Backup → Backup To command instead.