Interesting links which reveal that users have experienced poor performance when this quirk is active (and some ideas about how to disable the quirk, no idea if these methods will need to be modified for OpenWRT).
Note that depending on your particular USB drive, the quirk may be preventing some other bad behavior from the device; however, it would appear from these reports that some devices don't need it to be in place.
I have no idea if there's a fix for this in present-day kernels, which (if it exists) may not have made it into OpenWRT kernels yet.
https://www.mail-archive.com/debian-bugs-dist@lists.debian.org/msg1746447.html