[gentoo-user] why is writing to an async-mounted USB disk so slow?

[n952162]

I'm getting 13k bps writing to a Toshiba 3T drive over a USB/Sata
connection.  It' take 33 hours to transfer a 200meg file.

I mounted it async but then see that ext4 ignores the async option on
mounts.

What am I doing wrong?