public static void TestDigit(String file) {
			Console.WriteLine(file);
			var fs = new FileStream(file, FileMode.Open);
			var a = new Byte[fs.Length];
			fs.Read(a, 0, a.Length);

			Int32 cnt = 0, size = a.Length;
			var sw = Stopwatch.StartNew();
			//for (Int32 i = 0; i < size; i++) {
			//	var b = a[i];
			//	if (b >= 48 && b <= 57) cnt++;
			//}
			//Console.WriteLine($"if(b >= 48 && b <= 57) {sw.ElapsedMilliseconds} мс.  cnt: {cnt}");
			//cnt = 0;
			//sw.Restart();
			for (Int32 i = 0; i < a.Length; i++) {
				var b = a[i];
				if(b >= 48 && b <= 57) cnt++;
			}
			Console.WriteLine($"if(b >= 48 && b <= 57) {sw.ElapsedMilliseconds} мс.  cnt: {cnt}");
			cnt = 0;
			sw.Restart();
			for(Int32 i = 0; i < a.Length; i++) {
				var b = a[i];
				if(b >= '0' && b <= '9') cnt++;
			}
			Console.WriteLine($"if(b >= '0' && b <= '9') {sw.ElapsedMilliseconds} мс.  cnt: {cnt}");
			cnt = 0;
			sw.Restart();
			for(Int32 i = 0; i < a.Length; i++) {
				var b = a[i];
				if(Char.IsDigit((Char)b)) cnt++;
			}
			Console.WriteLine($"if(Char.IsDigit((Char)b)) {sw.ElapsedMilliseconds} мс.  cnt: {cnt}");
		}

  IL_0084:  ldloc.0
  IL_0085:  ldloc.s    V_6
  IL_0087:  ldelem.u1
  IL_0088:  stloc.s    V_7
  IL_008a:  ldloc.s    V_7
  IL_008c:  ldc.i4.s   48
  IL_008e:  blt.s      IL_009a
  IL_0090:  ldloc.s    V_7
  IL_0092:  ldc.i4.s   57
  IL_0094:  bgt.s      IL_009a
  IL_0096:  ldloc.1
  IL_0097:  ldc.i4.1
  IL_0098:  add
  IL_0099:  stloc.1
  IL_009a:  ldloc.s    V_6
  IL_009c:  ldc.i4.1
  IL_009d:  add
  IL_009e:  stloc.s    V_6
  IL_00a0:  ldloc.s    V_6
  IL_00a2:  ldloc.0
  IL_00a3:  ldlen
  IL_00a4:  conv.i4
  IL_00a5:  blt.s      IL_0084

			Byte[] a;
			using (var fs = new FileStream(file, FileMode.Open)) {
				a = new Byte[fs.Length];
				fs.Read(a, 0, a.Length);
			}

if(b >= 48 && b <= 57) 1202 мс.  cnt: 965430428
if(b >= '0' && b <= '9') 1203 мс.  cnt: 965430428
if(Char.IsDigit((Char)b)) 1159 мс.  cnt: 965430428

if(b >= 48 && b <= 57) 781 мс.  cnt: 965430428
if(b >= '0' && b <= '9') 740 мс.  cnt: 965430428
if(Char.IsDigit((Char)b)) 1435 мс.  cnt: 965430428

if(b >= 48 && b <= 57) 775 мс.  cnt: 965430428
if(Char.IsDigit((Char)b)) 1247 мс.  cnt: 965430428
if(b >= '0' && b <= '9') 768 мс.  cnt: 965430428

                var row = new DataRow();
                while (xml.Read()) {
                    if(find_item) {
                        if (xml.Name.Equals("group", StringComparison.Ordinal)) {
                            find_item = false;
                            row.value1 = xml.GetAttribute("value1");
                            row.value2 = xml.GetAttribute("value2");
                            ...
                            row.value9 = xml.GetAttribute("value9");
                    } else {
                        if (xml.Name.Equals("subdata", StringComparison.Ordinal)) {
                            row.value10 = xml.GetAttribute("value10");
				r = new DataRow(row); // копия для добавление в Dictionary
                            // тут убрано добавление в Dictionary
                        } else if (xml.Name.Equals("group", StringComparison.Ordinal) && xml.NodeType == XmlNodeType.EndElement) {
                            find_item = true;
                        }
                    }
                }

			var sw = Stopwatch.StartNew();
			Byte[] a;
			using (var fs = new FileStream(file, FileMode.Open)) {
				a = new Byte[fs.Length];
				fs.Read(a, 0, a.Length);
			}
			Console.WriteLine($"{file} - {sw.ElapsedMilliseconds} мс");