java string permutations and combinations lookup

I'm writing an Android word app. My code includes a method that would find all combinations of the string and the substrings of a 7 letter string with a minimum of length 3. Then compare all available combination to every word in the dictionary to find all the valid words. I'm using a recursive method. Here's the code.

// Gets all the permutations of a string.
void permuteString(String beginningString, String endingString) {
    if (endingString.length() <= 1){
        if((Arrays.binarySearch(mDictionary, beginningString.toLowerCase() +   endingString.toLowerCase())) >= 0){
            mWordSet.add(beginningString + endingString);
        }
    }
    else
        for (int i = 0; i < endingString.length(); i++) {
            String newString = endingString.substring(0, i) + endingString.substring(i + 1);
            permuteString(beginningString + endingString.charAt(i), newString);
      }
}
// Get the combinations of the sub-strings. Minimum 3 letter combinations
void subStrings(String s){
    String newString = "";
    if(s.length() > 3){
        for(int x = 0; x < s.length(); x++){
            newString = removeCharAt(x, s);
            permuteString("", newString);
            subStrings(newString);
        }
    }
}

The above code runs fine but when I installed it on my Nexus s I realized that it runs a bit too slow. It takes a few seconds to complete. About 3 or 4 seconds which is unacceptable. Now I've played some word games on my phone and they compute all the combinations of a string instantly which makes me believe that my algorithm is not very efficient and it can be improved. Can anyone help?

public class TrieNode {
TrieNode a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, z;
TrieNode[] children = {a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x, y, z};
private ArrayList<String> words = new ArrayList<String>();