Unzip the map and move the zip file elsewhere (otherwise, no matter how many changes you do, the game will read the zip instead of the folder; btw, if you don't play on a multiplayer server, the map can stay unzipped.)
Now in mapUS/multifruit/foliage (the crops are in the foliage inside the multifruit folder, the foliage from mapUS, has only 2 deco foliage from DJ Modding) you will find all the foliage xmls like wheat.xml, etc. Open each one of the crops that need changing and search for numBlocksPerUnit= and change the number inside ("4" for example can be"3", - but going lower than 3 might make that crop look patchy; won't affect the yield though, just the appeareance). Save the xml, open the game and see if it impoved the fps.
Of course you can always delete the folder and put back the zip if you messed up the xml.
The odd thing for me is that I expected my multifruit to cause problems on fps, not base-game...