This takes 2 mins 9 secs, and I don't know why:
time openssl rand 10000000 > foo
time openssl rand 10000000 | sponge > foo
Usbmount defaults to a 'sync' mode which makes some categories of write slow:
/etc/usbmount/usbmount.conf file and taking out the 'sync' property makes it as fast as you'd expect. Specifically the sponge and non-sponge tests having the same time.