aluminum dissipates heat faster
This is absolutely, but only partially, true.
Think about it though..... Heat, being energy, is neither created nor destroyed, at least not on the scale we're working with in an IC engine. Rather, all that can possibly happen, is that the heat energy changes to some other form of energy, or moves from one place to another. So what does "dissipate" mean? IOW, the metal "dissipates" heat from
It is more correct to say that aluminum conducts
heat better. Using that more accurate word, should make it clearer: it allows heat to flow from the combustion chamber and the exhaust port, through the casting, and into the coolant, more rapidly.
So, what you're seeing is absolutely totally normal and to be expected.
Also, over time, the buildup of carbon and whatnot on the surfaces exposed to hot gases, will create an extra insulating layer, and slow down the heat conduction somewhat. You could use ceramic coatings on those parts to slow the process down. But, as the engine gets run, it will run gradually cooler all by itself.
As a rough "rule of thumb", about 1/3 of the combustion energy goes into mechanical motion (this is the useful part); about 1/3 goes into the cooling system and is wasted; and about 1/3 goes out the exhaust as waste. Of the 1/3 that goes into the cooling system, about 40% escapes directly out the combustion chamber, about 40% passes through the exhaust port walls, and the rest gets there by all other routes (through the cyl walls, hot oil from the bottom of the pistons, etc. etc.) Anything you can do to keep the energy in the cyl and make it do mechanical work instead of being thrown away those other 2 ways, increases power output and efficiency at the same time.
The "slow down the coolant" thing is one of those myths that won't die.
Think about THAT for a minute... you've got a hot piece of metal, that you've been welding on or something let's say, and you need to cool it. Which way will it get cooler faster: if you plunge it into a bucket, or if you turn a fire hose on it?
How is an engine block any different?
Heat flows into the coolant in the block according to the temperature difference between the castings and the coolant, and then flows from the coolant into the air at the radiator, according to the temperature difference between the coolant and the air. Flow velocity through the system is not a factor. All that slowing down the flow will do, is to allow the coolant to reach a higher temp while in the castings, thereby DECREASING the flow rate of heat INTO it and causing the casting temp to INCREASE. Not the right answer, EVER, for anything (except if you live someplace cold and you want your heater to blow warmer air). It WILL NOT increase the total heat transferred from the castings to the air. In fact, unless MORE AIR is moved through the radiator, slowing the flow down will DECREASE the total heat moved by the cooling system.... exactly the opposite of the desired effect.
We have all these laws of physics and common sense and everyday experience and all that, that tell us how the world works. It's amusing how all of that wonderful logic and reason and rational observation gets pushed aside and instead people turn to myths, legends, mysticism, and voodoo, whenever an engine gets involved.