Now, this is just a suggestion based on my opinion. I think it is well known that french eco is just insane in treaty, probably the best and by far and i always though that this problem was never addressed correctly by removing food silos card for example, i think they can have it back if they want, the problem i believe to be the Northwest Passage card mostly but also CDB higher (+12.5%) speed (4.50) compared to almost any other vill in the game (4.00) with the exception of probably inca with 4.20 wich is not that different. I honestly don´t know why i´ve never seen this suggested before, i don´t know if i haven´t find them or if they are not really there, just one time i have seen it but just as a question and with not too many emphasis. Well, sorry for the long introduction, now, i don´t care about the 3x bonus to treasure guardians granted to CDB, just the +20% speed; the thing is, that in Mills and Plantations (wich are like the whole treaty eco except for factories) are defined by their “two-phase” gathering rate (asian civs don´t have this feature for example) and these two phases are the “gathering phase” and the “walking phase” wich translates in time (as i like to measure them) as the “gather time” and the walking time" (i´m sorry if i explain this like no one has ever played the game i just want to be clear xd). Now, the exact behavior of these to phases and the relation between them escapes to my knowledge and if someone know, please let us know too, but by reading some empiric tests by the community (thanks) and doing a little of my own (unsuccesful, i though there was a patern or a time, and maybe there is but couldn´t find it on the short time i test it) i can say that the average “gather time” of a mill or plantation (don´t know if they have separate and different walking times therefore gathering times but i handle them as equal, if they are equal then the opportunity cost of gathering coin in the plantation rather than food in the mill is just about 10.2% with all eco cards in contrast with the asian civs wich is almost 21% but anyway) is about 73% of the total cycle of gathering and walking (this, of course, varied from 70% to 76% depending on the test so it´s only average).
So, to sum up, my point is, that as the walking times of the mill and plantation are determined by the walking speed of the villager (obviosly xd), the 73% average of gathering phase is only on the case of the normal villager/settler, but the CDB has 5.40 speed with the Northwest Passage card instead of the normal 4.00 speed, so as it is 35% faster than normal villagers (5.40/4.00), the walking time is reduced by the same amount, and ends up with just 20% walking time (0.27/1.35) instead of the normal 27%, so that means there´s an 80% gathering time in the gathering phase wich translates to almost a 9.6% faster gathering rate overall or a 1.096 times the gathering rate of a nation with all eco cards (2.55 for food and 3.10 for coin) that if it doesn´t sound like a lot, it´s not like the economic theory card because it affects the base gathering rate and not the total like this bonus affects; just to put it in context, a nation with all eco cards except for the economic theory card, would benefit from this card by 4% for food (2.55/2.45) and 3.3% for coin (3.10/3.00) and this card (Northwest Passage) gives +9.6% to food and coin too…
One of the other reasons for their insane eco but also concerns military power is their thoroughbreds card but thats for another topic xd
So, thanks for all your attention and time, and whatever you like to post here is welcomed, whether it is a suggestion, a question if something isn´t clear or don´t know what i´m referring to, or if it is a criticism or just to mark an error in anything, whatever, is welcome.
Have a nice evening!