I agree with Greg here, ammonium dichromate isn't slower than potassium dichromate - given the solutions have the same dichromate ion concentration. When mixing by weight/volume, 1g A.D. is equal to 1.167g of P.D. - that is, A.D. is more sensitive per given weight. (Because you'll end up with more dichromate ions per weight.)

Regards,
Loris.