Large strings in ADA

[Johan Groth <jgroth@xpress.se>]

Hello,
I'm trying to write a program that converts a couple of types to strings
and are concatenated into one large string that is at the moment a
unbounded_string but when the string gets about 50KB big it takes longer
and longer to append to it.
The code looks like below. I need to concatenate about 2.5MB of data.
What is the fasted way to do that in ADA? Just for comparison a similar
program in C takes about one second. 
Can anyone help me?

TIA,
Johan

procedure Main is
   type String32 is
     record
	Info : String(1..32) := (others => ' ');
	Len : Natural range 0 .. 32 := 0;
     end record;

   Str : String32;
   No : Basic_Integer;
   Flt : Basic_Float;
   Msg : Unbounded_String := Null_Unbounded_String;
   Last : Natural;

   procedure Append_To_Msg (Str : in String;
                            Max_Len : in Natural;
                            Message : in out Asu.Unbounded_String;
                            Last : in out Natural) is
   begin
      Append(Message, " ");
      Append(Message, Natural'Image(Max_Len));
      Append(Message, " ");
      Append(Message, Str);
      Last := Asu.Length (Message);
   end Add_To_Message;

begin
   Str.info(1..6) := "hejsan";
   Str.Len := 6;
   No := 10;
   Flt := 5.5;
   for I in 1 .. 1600 loop
      Append_To_Msg(Str.Info, Basic_Natural(Str.Len), Msg, Last);
      if (I mod 100) = 0 then
         Put(" " & Integer'Image(I));
      end if;
   end loop;
   Put_Line("finished");
exception when Str:
  others =>
   Put_Line("exception in main: " & Ada.Exceptions.Exception_Name(Str));
end;


-- 
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           Johan Groth <jgroth@xpress.se> Kupolen Data