When there are volunteers for something, the tendency is for them to not work as hard. Seems natural. They are not getting any money for finishing the job faster, so why should they try?
I will say, I think this is more the case the more business-y you get. When doing humanitarian or church work, you often see many volunteers who are working extremely hard. Simply because these people are often doing things that give them more joy than getting money anyway.
But if you are in a more business setting and work extremely hard going the extra mile in that “volunteer” position, you will almost certainly stand out. Even people who don’t get paid much will have a bad mood that will prevent them from doing good work.
When you can work for free and not act like you’re working for free, you will make some very happy employers.