There is currently a glitch whereby you can increase the speed of a harvester to its maximum speed whilst harvesting. This will considerably reduce the amount of time this achievement takes.
You will need to hold
+
+
to activate cruise control then, whist still holding those buttons, hold
+
. After a few seconds your speed will increase from the default harvesting speed to the maximum speed for that vehicle.
Keep holding those buttons!
This will make it a bit difficult to steer as you are continually holding
but will drastically reduce the amount of time it takes to harvest a field!
This glitch was originally found on PS4, not by me, (video below) but i have replicated it successfully on Xbox numerous times.