by Jerry Lau.
Had the same here with 3.4.6+ and even a 1.2 GB backup took a long time and its just a specific one I wanted to test to see how long it would take via CLI method. Users reported it took so long they quit.. note that I have turned off cron.php just in case so nothing is running that may have an effect with our db server's computing resources.
I came across a setting for automated backup but I can't remember where to turn it off/disabled other than in scheduled task. The other setting was a drop down menu.. can't remember where I saw or how I came across it.. maybe through backup report.