As I recall, this is how some of the MasterChef perks work. One of them reduces meat usage by 20% (meaning 4 pieces instead of 5 for most recipes), another shortens the cooking time. Here's an excerpt from the code for the latter, from the progression.xml
<passive_effect name="CraftingTime" operation="perc_add" level="1,5" value="-.1,-.5" tags="perkMasterChef"/>
For each rank of Master Chef, you get 10% off the cooking time, starting at 10% off and going as high as 50% off.
That could act as a sort of guide for you. If you wanted to increase the crafting time, you could make those numbers positive instead of negative. The only problem is that in this case, it only affects things tagged with "perkMasterChef".
xpath modding isn't as daunting as it seems, and I'd recommend just jumping in and taking a stab at it. Mod something small first and you'll be used to it before you know it.