When add 1 lootupgrade, it will add 0.5 item in average for each harvest. When add 1 speed upgrade, it will make harvest faster than 1.5/2/3 times.

So the harvest item count in average during fixed_time is, for $X$ means the count of looting upgrade, $F$ means speed for upgrade:
$\mathbb{E} = f(x, F) = (1+\frac{1}{2}x)F^{3-x}/fixed\_time$。
For x from 0 to 3 and F from 1.5 to 3, the maximum value is always at x equals 0.

So, the loot upgrade is useless. How about making multiple speed upgrades the speed times add on each other instead of multiple?