A good onscreen keyboard, such as Swype, can be fairly fast... but horribly horribly inaccurate so I need to spend more time correcting the message than entering it.
The same is true of speech recognition. Quick, but so inaccurate as to waste more time fixing the resultant text than just typing it by hand would have been.
Source: I broke my right arm in mid-April and have spent a LOT of time and money looking into text entry systems since, including the very latest available Apple and MS Windows offerings.
I did the test on my iPad, i reach around 70% of the typing speed compared to my favorite PC keyboard.
There are some Tablets that include a keyboard in the cover, so in the end you have the same functionality and much better usability.