dd *very* slow write speed on Mac OS X?

simply use /dev/rdiskX instead of /dev/diskX, like following:

$ time sudo dd if=~/Downloads/kali-2.1.2-rpi2.img of=/dev/rdisk2 bs=1m
Password:
7000+0 records in
7000+0 records out
7340032000 bytes transferred in 435.829733 secs (16841513 bytes/sec)

real 7m18.005s
user 0m0.039s
sys 0m5.829s
$

… about ~7G in ~7min, not bad huh?)