I recently upgraded a couple of Cisco 3850 switches and noticed that the TFTP transfer rate to get the Cisco IOS files to the switch were horrible (approximately 200Kbps) which took a LONG time to transfer the 250MB file required for these switches.  After trying a couple different TFTP clients, and finding nothing that worked, I dug into the settings on my preferred TFTPD32 software and found that changing the TFTP setting for “Use anticipation window of” to 4092 gave me about a 4x to 5x improvement in transfer speed.  Still slower than I would have liked but definitely tolerable now.