RT-AC68U - 16 mbps USB3 write speed - is that normal ?

Hi,

I have Seagate powered USB3 HDD connected to USB 3 port of my RT-AC68U..

When I copy files from my desktop to this HDD, it is very slow at around 16 mbps..

is this normal speed?

or else what needs to be done to increase this speed?

Please let me know..

-KK