TFTP is fine for LAN, but very inefficient for sending bulk data over the Internet.

To keep things as simple as possible, TFTP sends one packet at a time, waiting for a response before sending the next packet. If your ping is 50 ms, the that’s 20 whole packets per second. (That’s assuming zero packet loss, otherwise it will be even slower.)

For historical reasons, the packet size is locked to 512 useful bytes. That puts the transfer speed at 20 kB/s (160 kbps), which is faster than dialup but not by much.