This is a quick video on how to speed up TFTP transfers to Cisco 3850 and 9300 switches. IOS images have gotten to several hundred megabytes in size, so copying them over TFTP can take a long time using the default settings. Luckily this can be greatly improved by increasing the TFTP block size.
The command is:
3850(config)#ip tftp blocksize 8192
Source:
https://www.cisco.com/c/en/us/support/docs/wireless/5760-wireless-lan-controller/117636-technote-tftpfile-00.html