If you want speed, perhaps you should search for syllables or several letters. For instance, the string "tccvssqqqlllxxccvv" is unlikely to contain any english words because it's all consonants. You could have a list of exceptions to search for (for instance, "cvs"), which would be much smaller (and thus much quicker) then using the entire dictionary. You could also extend the concept to vowels (i.e. "aeuaeey" can contain no words because it contains vowels which are not i (a word).

It's definitely possible to process your dictionary by running it through a program to chop it up into syllables. (And by syllables, I mean a combination of n letters. The more letters you use the more accurate, and the slower the algorithm will be). Add to that a few common sense rules like the one about all consonants for 10 syllables probably not being a word, and it should work.