The better Chinese input methods achieve ~3 keystrokes per character on handheld devices. [1] IIRC average word length in English is around 5 or 6, vs. around 2 for Chinese. So the input time should be about the same.
The real bottleneck isn't the actual typing, but thinking of something to type --- and it's a lot easier to think of how to say something verbosely rather than concisely!
You're right that Chinese and English can be comparable in input time, provided that you're using something like t9 for the English.
However, you lose a ton of speed the moment that you hit a word that isn't in your t9 dictionary. Chinese input methods don't have that problem. So, the best case for English usually equals the average case for Chinese.
Of course, since texting is the primary communication mode for many young Chinese, they can usually text blinding fast in any language.
The real bottleneck isn't the actual typing, but thinking of something to type --- and it's a lot easier to think of how to say something verbosely rather than concisely!
[1] http://www.pascal-man.com/navigation/faq-java-browser/2009_S...