The only things that change on server restart are:
- Doors go to a closed state. The game should be able to handle all door states on the map and stream it to players when the door gets in their network bubble.
- Player, animal and zombie corpses get deleted. This is done continually and the timer should be set so that during normal gameplay it will not lag the server.
- Ruined items despawn. This also done regularly during server uptime.
- Cooked grenade timers reset. I doubt that this is what is slowing down the server as players accumulate cooked grenades. But you never know with DayZ.
- Bugged items in inventory/hand bugs get fixed. I'm guessing that the server in most cases has a non bugged player state and it is used authoritatively.
- Anything else?
In most cases where programs have diminished performance resulting from uptime it is a memory leak. Or a section of code could get triggered and for some reason it gets executed more than intended or it doesn't stop. Some temporary files the program is working with get too huge to handle.
To me it seems like a bug somewhere in the code or some cleanup that happens that could be done continually as the server is up.
Restarting software is a poor fix. Eventually the underlying issue in the code must get resolved. You can't run your server at peak performance because you have to account for the degradation.