Seriously, I've reduced all the crop textures to 16x16 pixels (8x8 per stage) and I'm still only getting 10 FPS on my farm with 8 fields each of Cotton and Cereal in view. This is a tad ridiculous.
Possible changes:
-Fewer Leaves per plant
-Fewer polygons overall (straight stems?)
-Fewer plants per field
-Maybe see if there's some sort of bug that makes crops excessively resource intensive?
It's not a simple problem, since it involves sacrificing a great deal of artwork and undoing much of the intended look of the crops in order to make the resource demands tolerable on lower-end machines, but as it is there is a soft-cap on how many fields you can farm based on how low a frame-rate you're willing to tolerate, and your GPU temperature.