You have probably noticed that any upgrade to a farm (or mine) adds a rather small increase per hour to its resource generation. Then using simple math (but incorrect assumptions) it can be shown that it will take many days to gain the resources you didn’t accumulate during the upgrade.
For example, a level 16 farm upgrade takes 15 hours and 25 minutes. The level 16 farm produces 4102/hr - so you effectively lose 63,239 production during the upgrade. After the upgrade the boost in production is only an extra 298/hour. So dividing one by the other, you are looking at 8 days, 20 hour until you start “earning” again from that farm. If you just multiply that by all your farms (I have 9) it might seem like a discouraging total.
However it is easy to miss how the effect of upgrading is cumulative. After you upgrade 1 farm, yes it is only producing a paltry increase - but if you continue with all your farms, by the time you’ve finished, you are pushing out a significant increase per hour. So much so - that the total resource loss of 9 farms going from 16 to 17 (569,152) - is recouped in just 12 days, 1 hour, 17 mins and 43 seconds.
Yes, you read it right. Upgrading all 9 farms (from 16 to 17 in this example) will have you pushing out an extra 2682/hour on your current production rate, recouping your investment in about 12 days. That’s a lot less than 8.8 days times 9 (or 79.6 days) as one might calculate without taking into account the accumulative production benefit.
Given the long term aspect of the game, with Talents, Leveling, Training and Battle Items taking a heavy toll on your resources as you advance in the game, it is worth that extra bit of patience upgrading your farms and mines to their max level.
From 17 to 18: 14 days, 4 hours, 30 mins and 38 seconds.
From 18 to 19: 17 days, 54 minutes and 50 seconds.
Here are the calculations for 9 farms upgrading from 16 to 17.