You should remember that the data is transferred to the destination,
only as fast it is being read off the source.
USB 2.0 has a maximum theoretical speed limit of 60 MB/s. Actually, it
is only today, 8 years after the launch of USB 2, that USB 2 devices
are able to fully exploit this speed. Most of the USB 2 devices today
max out at 30-35 MB/s. The reason for this, the read-write speeds of
devices bottlenecks the transfer process.
Of course USB 3.0 isn't all about speed. The new bus specification is
also intended to accommodate the next generation of power hungry
devices, gadgets and drives.
So for USB 3, don't expect speeds of 400 MB/s plus; even though the
data could be transferred at that rate, it cannot be read off the
source HDD or written to the external drive at the same rate. Now if
you were to hook up a superfast SSD to the dock, & also had a similar
speed SSD in your system, then the transfer process would be
definitely faster, since both drives could read & write lighteningly
fast. So it is more likely that the transfers you are doing are
bottlenecked by the storage drives, in particularl, their sequential
read/write speeds; not due to the fault of USB 3.