環境:
PentiumM 2.00GHz
Memory 1GB
Delphi7
Windows XP Pro SP2
メモリ使用量などは見ていません。確保開放の速度だけです。

テストコード

//文字列結合
procedure TForm1.Button1Click(Sender: TObject);
const
  Cnt: Integer = 100;
var
  a, b: String;
  i, j: Integer;
  d: DWORD;
  ar: array[1..100] of DWORD;
begin
  a := 'abcdefghijklmnopqrstuvwxyz';
  for i := 1 to Cnt do begin
    b := '';
    d := timeGetTime;
    for j := 0 to 1000000 do begin
      b := b + a;
    end;
    ar[i] := timeGetTime - d;
    Memo1.Lines.Add(IntToStr(ar[i]));
  end;
  d := 0;
  for i := 1 to Cnt do begin
    d := d + ar[i];
  end;
  Memo1.Lines.Add('平均: ' + FloatToStr(d / Cnt));
end;

//StringList1
procedure TForm1.Button2Click(Sender: TObject);
const
  Cnt: Integer = 100;
var
  SL: TStringList;
  i, j: Integer;
  d: DWORD;
  ar: array[1..100] of DWORD;
begin
  for i := 1 to Cnt do begin
    d := timeGetTime;
    for j := 0 to 1000000 do begin
      SL := TStringList.Create;
      SL.Add('HOGE');
      SL.Free;
    end;
    ar[i] := timeGetTime - d;
    Memo1.Lines.Add(IntToStr(ar[i]));
  end;
  d := 0;
  for i := 1 to Cnt do begin
    d := d + ar[i];
  end;
  Memo1.Lines.Add('平均: ' + FloatToStr(d / Cnt));

end;

//StringList2
procedure TForm1.Button3Click(Sender: TObject);
const
  Cnt: Integer = 100;
var
  SL: TStringList;
  i, j, k: Integer;
  d: DWORD;
  ar: array[1..100] of DWORD;
begin
  for i := 1 to Cnt do begin
    d := timeGetTime;
    for j := 0 to 100 do begin
      SL := TStringList.Create;
      for k := 0 to 10000 do
        SL.Add('HOGE');
      SL.Free;
    end;
    ar[i] := timeGetTime - d;
    Memo1.Lines.Add(IntToStr(ar[i]));
  end;
  d := 0;
  for i := 1 to Cnt do begin
    d := d + ar[i];
  end;
  Memo1.Lines.Add('平均: ' + FloatToStr(d / Cnt));
end;