Optimize StartsWithPrefix

-Remove double "equality" test in StartsWithPrefix
-Further StartsWithPrefix efficiencies
-Remove subkey allocation in StartsWithPrefix

src/Microsoft.AspNetCore.Mvc.Abstractions/ModelBinding/ModelStateDictionary.cs

@@ -681,55 +681,52 @@ namespace Microsoft.AspNetCore.Mvc.ModelBinding
                 throw new ArgumentNullException(nameof(key));
             }
 
-            if (StringComparer.OrdinalIgnoreCase.Equals(key, prefix))
-            {
-                return true;
-            }
-
-            if (key.Length <= prefix.Length)
-            {
-                return false;
-            }
-
-            if (!key.StartsWith(prefix, StringComparison.OrdinalIgnoreCase))
-            {
-                if (key.StartsWith("[", StringComparison.OrdinalIgnoreCase))
-                {
-                    var subKey = key.Substring(key.IndexOf('.') + 1);
-
-                    if (!subKey.StartsWith(prefix, StringComparison.OrdinalIgnoreCase))
-                    {
-                        return false;
-                    }
-
-                    if (string.Equals(prefix, subKey, StringComparison.OrdinalIgnoreCase))
-                    {
-                        return true;
-                    }
-
-                    key = subKey;
-                }
-                else
-                {
-                    return false;
-                }
-            }
-
-            // Everything is prefixed by the empty string
             if (prefix.Length == 0)
             {
+                // Everything is prefixed by the empty string
                 return true;
             }
+
+            if (key.Length < prefix.Length)
+            {
+                return false;
+            }
+
+            var subKeyIndex = 0;
+            if (!key.StartsWith(prefix, StringComparison.OrdinalIgnoreCase))
+            {
+                if (key[0] == '[')
+                {
+                    subKeyIndex = key.IndexOf('.') + 1;
+
+                    if (string.Compare(key, subKeyIndex, prefix, 0, prefix.Length, StringComparison.OrdinalIgnoreCase) != 0)
+                    {
+                        return false;
+                    }
+                    else if (prefix.Length == (key.Length - subKeyIndex))
+                    {
+                        // prefix == subKey
+                        return true;
+                    }
+                }
                 else
                 {
-                var charAfterPrefix = key[prefix.Length];
+                    return false;
+                }
+            }
+            else if (key.Length == prefix.Length)
+            {
+                // key == prefix
+                return true;
+            }
+
+            var charAfterPrefix = key[subKeyIndex + prefix.Length];
             switch (charAfterPrefix)
             {
                 case '[':
                 case '.':
                     return true;
             }
-            }
 
             return false;
         }