Will you explain why a co-worker might think that you were doing less work if you leave early?
Maybe you can work something out with your supervisor, where if you finish your tasks for the day, you can leave when you are done. If you are in the US and on exempt pay, it won’t impact your salary. If hourly paid, your salary will be based upon the hours that you work. And if you don’t care about getting a full 40 hour’s pay, this arrangement could be a win-win situation.
Breaks are necessary, and it is not up to any of us to judge how that allotted time is spent.
Sometimes the reason people work more slowly than others is because they want to put a lot of care and thought into their work. They may also have other tasks that need to be completed and have a higher priority that you are not aware of. There will always be people who move both literally or figuratively faster or slower than you do. It’s part of life.
If these feelings are strong and not something you can learn to deal with and accept, I suggest that you look into another working environment. You are probably better suited for something where you are responsible for completing tasks/projects in a timely manner and do not rely on others. And preferably something you can do on your own.
I was a hotel inspector with a territory, and we predominantly worked alone and at our own pace, other than meeting goals for the # of inspections conducted. Family and friends have started their own business, which allowed them to work their own hours and schedules.