You can do this in game! While in game log in as admin on your server then press escape >> settings >> server settings. Towards the bottom of the list should be a crafting section where you can modify the settings. lower is faster btw. So 0.5 is x2 as fast for crafting. Once you are done making changes click apply and it should take effect immediately
I have been batteling low server fps. 15 or 14 when there are only a few people online but it dips down to 6-10 fps when more people join. What do you all get with your servers?