I am writing a program that counts the number of words in a text file which is already in lowercase and separated by spaces. I want to use a dictionary and only count the word IF it's within the dictionary. The problem is the dictionary is quite large (~100,000 words) and each text document has also ~50,000 words. As such, the codes that I wrote below gets very slow (takes about 15 sec to process one document on a quad i7 machine). I'm wondering if there's something wrong with my coding and if the efficiency of the program can be improved. Thanks so much for your help. Code below:

public static string WordCount(string countInput)
        {
            string[] keywords = ReadDic(); /* read dictionary txt file*/

            /*then reads the main text file*/
            Dictionary<string, int> dict = ReadFile(countInput).Split(' ')
                .Select(c => c)
                .Where(c => keywords.Contains(c))
                .GroupBy(c => c)
                .Select(g => new { word = g.Key, count = g.Count() })
                .OrderBy(g => g.word)
                .ToDictionary(d => d.word, d => d.count);

            int s = dict.Sum(e => e.Value);
            string k = s.ToString();
            return k;

        }