Again, I would say the right way to fix all the problems you mentioned is to limit the number of employees per farm, instead of slowing the work down. It eliminates the problem of hogging other people's harvest/plow, and who cares who use cheat programmes if there is no competition in a field? Employers will even be glad that fields are being plowed so quickly.
On a further note, how is it possible to distinguish cheaters from fast plowers? The speed of the fastest plower I've ever seen is quite reasonable i.e. reachable by manual clicking if the computer is good enough.