Yes. Access 2000 and 2002 were horrible with the
"track name autocorrect" and the
"sub data sheet issue" and the
"persistent lock".
These are discussed in the first couple bullets of the first link. The name autocorrect and the subdatasheet issue alone would make the database unuseable. Apply those first three at a minimum. The rest become an art. There are hundreds a little things to try as these post list. Most of the time these do not make a huge difference, but every once in a while one of these little fixes will make a huge difference
Running on a network is almost always going to be slower, depending on the speed of the network. Applies these and see what happens. This may be as good as your going to get. Try a simpler database on the network and see if you get OK performance.
The other thing, the form and code is not optimized. The code took the simple route. When you update a record or change the date it basically reloads the form instead of just updating the record in the "grid table". This could be modified so that when you add/edit/delete a shift it just updates the record in the "grid table". The code would be more complex, but pretty similar to the existing write to grid code.
If other forms work OK and it is only this one with the performance problems, then I would look at modifying the code. If all forms are suffering then spend the time on these other fixes.