Take a break! Take vacation for a week and let them see how little gets done, when you're not here to do it.

My wife has a co-worker who is exactly like this. He doesn't have enough to do as it is and has made his job pretty efficient so he has lots of spare time in his day. Problem is that he ONLY does his job and does nothing to help anyone else - ever. He hides behind a stack of boxes playing on his phone when he does not have any of his own work to do. When asked to help out with side projects he suddenly becomes busy and can't help. Everyone else pitches in with extra cleaning, stocking, inventory tracking etc but he just hides in the back and refuses to be a team player.

Start recording what you do. It probably looks like you do nothing to them. Write down times it took, and at the end of the week show it to your supervisor. If you all have equal work, you should not have to pick up slack because they're lazy. Unfortunately, working fast has disadvantages too. :(

OP, make a note of this and save this anecdote for your next job interview. Of course you bring it up in a polite, professional way, but it's great to know you work efficiently and quickly. Too many people lie about that on their resume. I would certainly hire you or at the least give you a shot at a position with that work ethic.

Maybe OP has made this a one sided story and believes they do the work and really don't (don't get me wrong, the thought came to my mind though)

This is sad.. Unfortunately its very common some even say always look busy and walk around with papers to give the impression of being bust and stressed!!! Stupid ? Yes. But happens a lot..

Next time you finish your work early, try making rounds and asking your coworkers if you can help them with anything, since you have finished your own work. This will show that you are taking initiative, and people who didn't see you working won't get the impression that you're sitting around doing nothing.

Seems to me some of your coworkers are jealous of you.

And then from there you show what you completed...problem solved...

This has happened to me at quite a few jobs. I work too fast and they either accuse me of doing things half assed or they get mad that I'm making others look bad.