Running time take its too long for adding and finding string from txt C#

static List<string> getDBList(string DBname)
{
     List<string> listWords = new List<string>();
     string[] files;

     try
     {
         files = Directory.GetFiles(@"dbase/", DBname); 
         foreach (string file in files)
             foreach (string line in File.ReadAllLines(file))//doubt
                listWords.Add(line.Trim().ToUpperInvariant());
     }
     catch (Exception ex)
     {
         Console.WriteLine(ex.ToString());
         return new List<string> { };
     }

     return listWords;
}

//MAIN PROGRAM
string allInput = rtbInput.Text;

List<string> splitString = new List<string>.Split(new char[] { ' ', '\t', etc...});
List<int> AllIndexes = new List<int>();
HashSet<string> nounList = new HashSet<string>(getDBList("nounList.txt"));//doubt

int startIndexes = 0;

foreach (string s in splitString)
{
    if (s.Trim() != "")
    {
       string word = s.Trim();

       if(!(nounList.Contains(word.ToUpperInvariant())))   //doubt, if not found, color it
       { 
               tbTest.Text += word + " ";

               //index to begin color the text
               AllIndexes = WordsIndex(word, startIndexes);

               foreach (int item in AllIndexes) //Coloring all appearance of the word.
               {
                   tbSeeIndex.Text += Convert.ToString(" " + item + " ");

                   rtbInput.Select(item, word.Length);

                   startIndexes = item + word.Length;

                   rtbInput.SelectionColor = Color.Red;
              }

              tbL.Text += Convert.ToString(" " + startIndexes + " ");
        }
    }
}