UDP by itself cannot be used to transfer files or any other kind of data with a size bigger than an IP packet.
So it is impossible to compare the performance of TCP and UDP.
UDP is used to implement various other protocols, whose performance can be compared with TCP. Any protocol implemented over UDP must have a performance better than TCP, at least in some specific scenarios, otherwise there would be no reason for its existence.
I do not know how UDP is used by iperf3, but perhaps it uses some protocol akin to TFTP, i.e. it sends a new UDP packet when the other side acknowledges the previous UDP packet. In that case the speed of iperf3 over UDP will always be inferior to that of TCP.
Sending UDP packets without acknowledgment will always be faster than for any usable transfer protocol, but the speed for this case does not provide any information about the network, but only about the speed of executing a loop in the sending computer and network-interface card.
You can transfer data without using any transfer protocol, by just sending UDP packets at maximum rate, if you accept that a fraction of the data will be lost. The fraction that is lost can be minimized, but not eliminated, by using an error-correcting code.
> perhaps it [..] sends a new UDP packet when the other side acknowledges the previous UDP packet. In that case the speed of iperf3 over UDP will always be inferior to that of TCP
It does not, otherwise it would be impossible by a factor ~100x to measure 4.5 Gbit/s as per the bandwidth-delay calculation (the ping is around the usual 0.2 ms).
With iperf3, as with many other UDP measurement tools, you set a sending rate and the other side reports how many bytes arrived.
It is a long time since I have last used iperf3, but now that you have mentioned it I have also remembered this.
So the previous poster has misinterpreted the iperf3 results, by believing that UDP was slower, as iperf3 cannot demonstrate a speed difference between TCP and UDP, since for the former the speed is determined by the network, while for the latter the speed is determined by the "--bandwidth" iperf3 command-line option, so the poster has probably just seen some default UDP speed.
So it is impossible to compare the performance of TCP and UDP.
UDP is used to implement various other protocols, whose performance can be compared with TCP. Any protocol implemented over UDP must have a performance better than TCP, at least in some specific scenarios, otherwise there would be no reason for its existence.
I do not know how UDP is used by iperf3, but perhaps it uses some protocol akin to TFTP, i.e. it sends a new UDP packet when the other side acknowledges the previous UDP packet. In that case the speed of iperf3 over UDP will always be inferior to that of TCP.
Sending UDP packets without acknowledgment will always be faster than for any usable transfer protocol, but the speed for this case does not provide any information about the network, but only about the speed of executing a loop in the sending computer and network-interface card.
You can transfer data without using any transfer protocol, by just sending UDP packets at maximum rate, if you accept that a fraction of the data will be lost. The fraction that is lost can be minimized, but not eliminated, by using an error-correcting code.