I will say I downloaded one of those so called english word lists and most of them don't look like words to me, but anyways.
Here is a test to load such a wordlist into a dictionary without hanging the browser. Once loaded it's as simple as the "has key" dictionary condition.
dropbox.com/s/in4dx5x2z03yvqc/dictionary_load.capx
As far as speed goes, its way faster to have the words in a dictionary instead of an array if you need to find if a word is in it.
With an array it would have the worst case of having to look at every single word before finding it.
You can do a binary search since the words are sorted to improve that. Maybe a worst case of checking 40% of the words.
With a dictionary it finds the word more or less directly.