Clear["`*"]
mandel[a_, it_: 100] :=
  Module[{z = 0. I, c = a[[2]] + a[[1]] I},
   Catch[Do[If[Abs[z] > 2, Throw@i];
     z = z^2 + c;, {i, it}]; it]
   ];

mandelc =  Compile[{{a, _Real, 1}, {it, _Integer}},
   Module[{z = 0. I, c = a[[2]] + a[[1]] I},
    Catch[Do[If[Abs[z] > 2, Throw@i];
      z = z^2 + c;, {i, it}]; it]
    ], RuntimeAttributes -> {Listable}, RuntimeOptions -> "Speed"];

d1 = mandelc[Outer[List, Range[-1., 1, 2./199], Range[-2., 0.5, 2.5/199]], 100]; // AbsoluteTiming
d2 = Table[ mandel[{i, j}], {i, -1., 1, 2./199}, {j, -2., 0.5,  2.5/199}]; // AbsoluteTiming

d1 == d2
Colorize[#, ColorFunction -> Hue] & /@ {d1, d2}