Optimize String#== taking character size into account

src/string.cr

@@ -3088,8 +3088,18 @@ class String
   # "abcdef".compare("ABCDEF", case_insensitive: true) == 0 # => true
   # ```
   def ==(other : self) : Bool
+    # Quick pointer comparison if both strings are identical references
     return true if same?(other)
+
+    # If the bytesize differs, they cannot be equal
     return false unless bytesize == other.bytesize
+
+    # If the character size of both strings differs, they cannot be equal.
+    # We need to exclude the case that @length of either string might not have
+    # been calculated (indicated by `0`).
+    return false unless (@length == other.@length) || (@length & other.@length).zero?
+
+    # All meta data matches up, so we need to compare byte-by-byte.
     to_unsafe.memcmp(other.to_unsafe, bytesize) == 0
   end