I believe the LNURL zap NIP has a design problem at its root, at least how I read it. The LNURL provider needs to send proof of the zap to every client that reads the post, independent of any relay. This proof-of-zap should be saved on the relay by the zapper so other readers don't need to go out to the LNURL provider for each post. To do this adds an enormous load on the clients and on the zap provider and would discourage LNURL providers, particularly small ones, from participating. Who wants their RPi DDOSd each time your receive a zap?