You write the UDP payload onto PCI-Express, then the NIC writes it onto the wire as an Ethernet frame.
Obviously you're limited by PCI-Express bandwidth then Ethernet bandwidth. There is also a bit of jitter tied to the underlying protocols but those delays aren't really significant for most applications.
Meanwhile, TCP can queue data indefinitely, based on how the other end is sending acks and the size of the window.
And what about all the switches, routers, bridges, firewalls, and whatever else in between endpoints?
What a strange thing to require. It comes off a bit like X-Y. You seek high performance. But you state it as wanting zero copy networking. There’s a lot more to performance across the overall system than that.
I doubt this engine makes this guarantee, since it supports TCP.