In EXOS, the TFTP client is using the data block size of 1,400 bytes by default. (The block-size refers to data octets and does not include TFTP header.)
With the default block-size, a TFTP server sends a 1,400 byte packet to a client and waits until it gets an acknowledgement from the client before it starts sending a next one.
Due to the acknowledgement required for each packet from the client, the delay between the server and the client significantly contributes to the overall download speed.
Supposing that the round trip time is 100 ms, the server can at most send ten 1,400 byte packets within a second which are 14K bytes in total, no matter how high bandwidth a link supports.
At this rate of the server sending packets to the client, it would take the client a significant time to download a file over TFTP.