Xbox 360 DVD is
5X to 12X, I guess depending on where on the DVD the data is.
That
translates to 6.925 MB/s to 16.62 MB/s. Note that this is not "real world performance" but is what should be achievable generally.
External hard drive transfer rates
five years ago in real-world tests varied wildly but could on some models average at just under 30 MB/s.
The peak transfer rate (including data and control) of USB 2.0 is 60 MB/s, so the upper limit won't go wildly higher than what you saw in that aforelinked test, but I guess there might be drives with ~45 MB/s average rates in more recent times, if you'd care to look more aggressively than I did. I don't know what the typical speed for USB 2.0 is.
This may very well be a limiting factor on the Wii U, but the original assertion of USB 2.0 being slower than the Xbox 360 DVD is certifiably false.