According to the Mbed documentation socket.recv method should return NSAPI_ERROR_WOULD_BLOCK in case it non-blocking or timeout.

But if we check the code we can

From the original project:

I don't now if there is a difference, but with W5500, I was able to upload a 250k sketch to mega2560. it was so slow, so I reduced the #define TFTP_PACKET_DELAY to 20 ms. worth trying, I think..

Note from the author:

With W5500 there is a possibility that you could remove the delay completely as it is. You don't encounter these issues with it because W5500 i