6 hours for prep, and 6 hours for delivery, is without a doubt too slow.
I get the impression you aren't too bothered about your speed; rather you are more concerned as to whether you can lose your job as a result of it.
Can you lose your job for being too slow? Well, since you are willing to go out of your way and work for nothing the managers probably wont care. But yes, you COULD end up losing your job for being too slow if it became an issue and you couldn't improve your speed.
Is your manager aware of what time you're finishing? Has he ever discussed your speed with you? Have your colleagues ever commented on your speed?
It sounds like you may need to be coached to get you up to speed... but if it's just a case of you're just working too slow then coaching probably isn't going to do much for you. When you're prepping a walk you need to make sure you concentrate so you can familiarize yourself with the frame. I personally don't road letters, but I do know that some people find that it helps... if you find that roading letters first helps you learn the frame then ask your manager to get someone to road it for you for a while, WHILE you throw it off and get used to the frame. This is what was done for me while I was being trained.
If you're having trouble with prep and delivery you're probably also taking a long time to sort out your packets, do your redirections, band up etc, this is why it could be helpful if you have a coach watch you and advise you on how to improve.
If you want to get up to speed I don't see why you can't. But if you don't mind working until 6pm then you probably wont speed up... that's why I'm telling you straight that YES YOU ARE SLOW, AND BEING SLOW COULD JEOPARDIZE YOUR JOB.
I think rather than asking whether being slow can result in losing your job, you should ask for help on getting up to speed. What do you think?