Having owned and run various companies myself for the past 40 years ..... that sounds nothing like a 'salary' situation. A fixed hours contract .... Yes, definitely.
What does your contract say?
Normally 'salary' starts at low level management upward. It is a fixed amount but they may get bonus on profits. They do not get over time and usually they are worked to death.
or at least nervous breakdown.
However you do seem a tad naive to say the least .... If they want you to complete 40hrs a week, not work necessarily but be there 40hrs a week, then stop rushing the work you have and make it last ...... If you run out of work then sweep the frigging floor. Find something to do
because they ARE paying you. Adopt the outlook of the services ... If it moves salute it, if it doesn't move then pick it up and if you can't pick it up - paint it.
As you can see there are no medals for being 'fast', instead you are just irritating people.
If I were your employer I am certain you would not be leaving before your 8hrs each day ... Because for example, if you walk away at 15.00 but by rights should be there until 17.00 .... and I have work for you at 16.00, then I would not be best pleased.
Plus I would soon be of the opinion, that your job could be part time or 'flexible' and you only get paid when I need you .... Do remember matey no one is indispensable - No one, not even you.