Main problem is evaporation of the water that is on the inside wall. I tried measuring and calculating it, was about 1 g within something like 10 s (rate decreases and evaporation eventually stops when no more water, depends on conditions like room temperature, humidity etc) - can't mount the cylinder with tongs to PF and start pouring water much faster than that. It means temperature drop of that amount of steel around 10°C (temperature probe fitted between cylinder and silicone confirmed the calculation quite well). Tried covering the top while doing that, but didn't help much
Outside wall is less of the problem, silicone is a kind of insulator (though not especially good), exposed top of the cylinder doesn't get into direct contact with water (insulating it anyway didn't make a difference), so toweling it wouldn't help (you also need additional 2-3 seconds to do it). With steam preheat, it is dry anyway (assuming suitable vessel used)
When water is poured in, after some time temperature of the cylinder and water equalize, according to approximate formula 0.33 * Temp of cylinder + 0.67 * Temp of water (with assumption 60 g water 4.2 kJ/kgK, 250 g steel 0.5 kJ/kgK), minus of course other temperature losses. So if original preheat method used (which heats the cylinder to something like 70°C at best minus decrease due to water evaporation - luckily less than with boiling preheat because cylinder temperature isn't very high and outside is also dry), you have to be really quick to do most of extraction before water cools down below usual espresso brew range
Cylinder filled with water cools slower (water surface is much smaller than inner surface of the cylinder, water temperature now significantly lower than boiling point) and when plugged with piston, cooling is even more slow - only through convection from the outside surface.