uhh, so im not sure if you are still looking for help or anything.
But the .Json file with crop growth speed is in :
Drive the game is currently installed(In my case L: drive) Folder Vintagestory
Assets
Survival
Blocktypes
Plants
Crop
Find crop you want to edit
You can Easily right click on your desktop icon and click "open file location". I edited most of the crops to take about 2 weeks to fully mature other than flax, Flax takes 2 days because i hated waiting on everything