The title says it all, at some point of changing underground belt length it causes freezes than can last up to 5 whole minutes.

decrease the pathfinding speed?

Decrease path finding speed indeed can reduce lagging.

The root cause of the lag is because underground length = number of potential belt connections, so more underground length = more work to do in each step. I can’t see a good way to solve this issue, maybe set a maximum cap for number of underground connections to consider in each step?

In my case, I tested decreasing the path finding speed, from the default 5 decrementally to 1, It did help a tiny bit though ( gave me the ability to move my character but with painful lag), the only thing that fixed it was to default the underground belts lengths, I'm sorry I can't be of more help other than this.

did you try changing other values?

No