Fastest way to find objects from a collection matched by condition on string member

class ClassFoo
{
 public string word;
 public float score;
 //... etc ...
}

List<ClassFoo> result = new List<ClassFoo>();
foreach (ClassFoo cf in collection)
{
  if (cf.word.StartsWith(query) || cf.word.EndsWith(query))
    result.Add(cf);
}

Dictionary<char, List<ClassFoo>> indexByFirstLetter;
foreach (var cf in collection) {
  indexByFirstLetter[cf.bar[0]] = indexByFirstLetter[cf.bar[0]] ?? new List<ClassFoo>();
  indexByFirstLetter[cf.bar[0]].Add(cf);
  indexByFirstLetter[cf.bar[cf.bar.length - 1]] = indexByFirstLetter[cf.bar[cf.bar.Length - 1]] ?? new List<ClassFoo>();
  indexByFirstLetter[cf.bar[cf.bar.Length - 1]].Add(cf);
}

foreach (ClasssFoo cf in indexByFirstLetter[query[0]]) {
  if (cf.bar.StartsWith(query) || cf.bar.EndsWith(query))
    result.Add(cf);
}