Aluminum is, on average and depending on the specific alloys, about three times as heat conductive as iron, so it draws heat away three times faster, keeping the cavity temperature closer to the outside temperature of the block. Iron is about three times more dense than aluminum, but its heat capacity is only about half that of aluminum, so you are looking at a same-size aluminum block having 1.5 times more heat required to raise it each degree, as well as having 3 times the conductivity. Taken together, it is harder to warm up. Smoke makes a small insulation barrier, so it tends to stop heat from being drawn out quite so fast.
Be careful you don't warp the mold with hot plates and whatnot. Usually, if you set it across the top of your melt pot when you first start heating it, by the time the main mass is warm the blocks have been given a good start. Then just casting a few rounds fast will finish fringing it up to temperature. At least, that's my experience with the 6-cavity Lee .38 molds. The one that gave me trouble was the 6-cavity .32 wadcutter they used to offer. They've dropped it, and I expect mold heating problems were part of that.