You simply don't get the advertised throughput with USB 2.0 for file transfer speed. From e.g. here:
Actual data throughput is usually much less than the maximum advertised USB specification and is a function of many variables, including overhead. Actual throughput in practice is typically up to 35 - 40MB/sec for USB 2.0 [...]
The article links to Wikipedia, with even worse numbers:
[...] limited by the current number and type of attached USB devices and by the upper limit of the USB interface (in practice about 30 MB/s for USB 2.0 [...])
Typical hi-speed USB hard drives can be written to at rates around 25–30 MB/s, and read from at rates of 30–42 MB/s, according to routine testing done by CNet. This is 70% of the total bandwidth available.
The difference includes USB protocol overhead, and of course other connected devices that share the total available bandwidth. Chances are you have several peripherals connected via USB (even internal devices, such as trackpad or keyboard in a Macbook). You're so close to those values (values my experience also supports), that you probably shouldn't waste any more time but just start copying.
tar is useless in your case (unless your internal disk is even slower than USB 2.0), since the data read and transferred via USB is the same plain files as without. This trick usually only helps when transferring to another computer via SSH, where a slow network is the bottleneck.
The fastest way to transfer the data is to connect the disk using a fast interface, e.g. FireWire 800, or (not commonly on Macs) eSATA, or even Gigabit Ethernet. Don't use USB 2.0.