How big is the performance impact of this mod in a big save?

I don't have a "big save" so I don't have any data on that whatsoever. It will depend on the size of your network (in terms of things that requests stuff from the network (e.g. buffer/requester chests)), as it loops over all of them within the network in which the unfulfilled requests detector is placed.
This currently happens for each of them placed (even if they're located in the same logistic network, which is something that could be improved).

That sounds as it could impact performance really heavily.

Yes, unfortunately I have not seen another way to get the required information.