Did you use more weight to make up for the shorter time? I put a heavy book on top for twenty minutes and it turned out great... no cracks or anything. Honestly, short presses should be more common.
Twenty minutes with just a book on top? That's blowing my mind (in a totally skeptical way). Every guide I've ever seen says you need at least a few hours of press time for the glue to set right, even with quick-dry stuff. The physics of it just feels wrong, like the spine wouldn't get an even settle. I'd be scared to even open it, worried the whole thing would just sigh and fall apart. You must have some kind of magic touch (or a seriously heavy book).