# dd to USB stick on macOS too slow? be sure your target
# is the *r*disk; also, it might help to set your block
# size to 128kB, which is apparently the size limit for 
# the USB protocol.
#
# IMPORTANT: for the most part, dd is the *nix equivalent
# of a loaded firearm: it DOES NOT CARE what's on its target 
# volume and under most circumstances WILL OVERWRITE DATA 
# WITHOUT ASKING YOU; take great pains to determine which device node
# represents your desired target media; GRIMM IS NOT
# RESPONSIBLE FOR DATA LOSS DUE TO YOUR USE OF dd!

# e.g., to "burn" an ISO file called "downloaded.iso" to
# a USB thumb drive located at /dev/disk7:
[grimm@mac Downloads]$ sudo dd if=downloaded.iso of=/dev/rdisk7 bs=128k

# this information courtesy of http://daoyuan.li/solution-dd-too-slow-on-mac-os-x/
# and the superuser.com thread to which that links