好吧，代码太少了。我怀疑

DictionaryWords += ss;

是重罪犯追加字符串130000次，这意味着 * 重新创建 * 相当长的字符串 * 一遍又一遍 * 可以很好地让系统屈服，但我没有严格的证明（我在评论中询问了DictionaryWords）。
这就是为什么让我试着从头开始解决这个问题。

• 我们有一个SI-utf8.Txt的（长）字典。
• 我们应该在不冻结UI的情况下加载字典。
• 我们应该用这本词典来翻译英语课文。

我有这样的东西：

using System.IO;
using System.Linq;
using System.Threading.Tasks;

...

// Loading dictionary (async, since dictionary can be quite long)
// static: we want just one dictionary for all the instances
private static readonly Task<IReadOnlyDictionary<string, string>> s_Dictionary = 
  Task<IReadOnlyDictionary<string, string>>.Run(() => {
    char[] delimiters = { ' ', '\t' };

    IReadOnlyDictionary<string, string> result = File
      .ReadLines(@"SI-utf8.Txt")
      .Where(line => !string.IsNullOrWhiteSpace(line))
      .Select(line => line.Split(delimiters, StringSplitOptions.RemoveEmptyEntries))
      .Where(items => items.Length == 2)
      .ToDictionary(items => items[0], 
                    items => items[1], 
                    StringComparer.OrdinalIgnoreCase);

    return result;
  });

然后我们需要一个翻译部分：

// Let it be the simplest regex: English letters and apostrophes;
// you can improve it if you like
private static readonly Regex s_EnglishWords = new Regex("[A-Za-z']+");

// Tanslation is async, since we have to wait for dictionary to be loaded
private static async Task<string> Translate(string englishText) {
  if (string.IsNullOrWhiteSpace(englishText))
    return englishText;

  var dictionary = await s_Dictionary;

  return s_EnglishWords.Replace(englishText,
    match => dictionary.TryGetValue(match.Value, out var translation) 
      ? translation   // if we know the translation
      : match.Value); // if we don't know the translation
}

用法：

// Note, that button event should be async as well
private async void button1_Click(object sender, EventArgs e) {
  TranslationTextBox.Text = await Translate(OriginalTextBox.Text);
}

• • 编辑：**因此，DictionaryWords是string，因此

DictionaryWords += ss;

是一个 * felon *。请不要在（深度）循环中追加string：每个append * 重新创建 * 字符串，这是很慢的。如果你坚持循环，使用StringBuilder：

// Let's pre-allocate a buffer for 6 million chars
StringBuilder sb = new StringBuilder(6 * 1024 * 1024);

using (StreamReader sr = File.OpenText(fileName))
{
    while ((ss = sr.ReadLine()) != null)
    {
        sb.Append(ss);
    }
}

DictionaryWords = sb.ToString();

或者，为什么要循环呢？让. net来为您做这些工作：

DictionaryWords = File.ReadAllText(@"SI-utf8.Txt");

• • 编辑2：**如果实际文件大小不是 * 那么大 *（只有DictionaryWords += ss; * 一个人 * 会破坏乐趣），您可以坚持使用简单的同步解决方案：

private static readonly Regex s_EnglishWords = new Regex("[A-Za-z']+");

private static readonly IReadOnlyDictionary<string, string> s_Dictionary = File
  .ReadLines(@"SI-utf8.Txt")
  .Where(line => !string.IsNullOrWhiteSpace(line))
  .Select(line => line.Split(new char[] { ' ', '\t' },
     StringSplitOptions.RemoveEmptyEntries))
  .Where(items => items.Length == 2)
  .ToDictionary(items => items[0], 
                items => items[1], 
                StringComparer.OrdinalIgnoreCase);

private static string Translate(string englishText) {
  if (string.IsNullOrWhiteSpace(englishText))
    return englishText;

  return s_EnglishWords.Replace(englishText,
    match => s_Dictionary.TryGetValue(match.Value, out var translation) 
      ? translation 
      : match.Value);
}

那么用法就很简单了：

// Note, that button event should be async as well
private void button1_Click(object sender, EventArgs e) {
  TranslationTextBox.Text = Translate(OriginalTextBox.Text);
}