The xp gain is reduced the longer the timer is and the higher the gold cost is (it also was same way before the patch). your max level with a crop is also a factor and it seems like that if your crop is at the highest level it has ever been - you don't get any passive xp whatsoever (or extremly little, idk). However if you just prestiged a crop - it actually does climb up a couple of levels at a pretty decent pace. So previously everything gained exp constantly at the same rate but at an absolute snail's pace. Now it's a catching up mechanic for prestiged crops so that you don't have to start from 0. Though it would be nice to see actual xp gain for any individual crop since they're so drastically different from the number in the lab.
The description does not say anything about changing the speed depending on the level of culture. Yes, it does not make sense, since the amount of experience needed for the level itself is growing very quickly, which in total will provide the same drop in speed that you describe. Nevertheless, I want to note that over the next approximately 15 hours, my potatoes received 6 experiences, and 8 in a day. That is, literally an order of magnitude less than stated.
Notice, I'm not saying this is wrong. I'm saying it's not clear. How can I plan something on this mechanics if it works so unpredictably? Gives an unknown amount of experience with such a wild spread in speed during the day.
Note that the cryolab mechanics themselves work very differently for each tab. The mine has no overflow of crystals, so the maximum value for it is constant. And a passive increase of 2-4 percent to the growth rate will be an increase of 2-4 percent to the speed. The village and the horde have overflow, so just waiting once, say, 10 times longer, we can significantly increase the increase in speed. 3-4 times, which is much nicer. Well, the farm works like this. Gives it is unclear how much, all in different ways and at different speeds during the day. I don't know what will happen with the fifth tab, I haven't opened it yet :) And as a result, a cryolab working so differently looks a little strange. I will not say what is wrong, rather it requires special attention.