It does add a bit of time, but I was previously trying to record comments as much as possible anyway, so this does speed that up.
You’re right that the duplication of writing out comments in a book and typing into your own markbook isn’t very efficient. I’m working on a few ideas for doing feedback exclusively electronically, to remove this duplication. I think perhaps we need to get away from the idea that the student’s exercise book should be the primary (and perhaps only) record of the feedback they have received. Books can be lost, marking can be spread across multiple books, students whose work is messy can have difficulty using their books effectively.
I’m interested in your idea of using a Google Doc for each student where some of this stuff is recorded. I’m trying something different but similar, and I’ll report back when I’ve tested it out a bit more.