Hey there,
I've stumbled across some discussions in the past regarding modification in the vanilla bot pathfinding. Example, to make the bots do their job without leaving the logistic network, even if for that it wouldn't follow a straight line.
One example of such discussion is the following:
https://www.reddit.com/r/factorio/comments/6lta82/mod_request_limiting_bot_pathing_to_within/
In all occurrences of the discussion that I've stumbled upon, that idea has been turned down mainly because pathfinding is expensive, and would drop UPS significantly. And that does make sense, pathfinding is expensive, and the devs solution for bots to make only straight line paths, which is cheap to calculate, also makes sense.
Now for the question: How does Transport Drones not dramatically reduce the game UPS?
The question is more a curiosity and from a mod development perspective. I hope I didn't bore you too much with it.
Thanks for the great mod!