From personal experience, I am aware that heat issues on laptops are often caused by a poor application of the stock thermal paste (also known as “thermal interface material” or TIM), provided that the cooling system is functioning. The reason is simple: the thermal paste – as the name suggests – is supposed to facilitate the transfer of the heat from the CPU/GPU to the heatsink. This only works efficiently, though, if a very thin layer of thermal paste is applied between CPU and heatsink in such a way that minimises the chance of creating “air bubbles” (air has a bad thermal conductivity). So the problem is that very often, the stock thermal paste is applied in factories in ridiculously large amounts, that often spread out of the die of the CPU and that most certainly achieve the opposite effect by slowing down, instead of facilitating, the transfer of heat from CPU to heatsink. Sadly, Apple doesn’t seem to be any different from other manufacturers from this point of view, despite the higher prices and the generally wonderful design and construction quality. Plus, often the stock thermal paste used by some manufacturers is quite cheap, and not based on some very efficient thermally conductive material.
This is a very common problem, and one that is actually fairly easily rectified if you have even a modicum of understanding of how a screwdriver works. I’m planning on replacing the stick thermal paste on my XPS 13 9370 just to see if it will make a difference. I run Linux on it – KDE Neon – and Linux is slightly less efficient at decoding video than Windows, causing more fan spin-up. There’s a very real chance replacing the thermal paste will give me just enough thermal headroom to address this issue.