Top of the line laptops released this year are all relying on Intel's Core i9-8950HK CPU which is know to run very hot.
The first laptop released with this CPU that got my attention and I eventually bought, was the Dell XPS 9570. This laptop is known to have thermal issues since the last generation - 9560. This year generation didn't change much, it also throttles, but almost everything else about it is great (almost = webcam position still sucks).
The second laptop, the Macbook Pro 2018 with i9 is not only late to the party, but also severely throttling under heavy use. It's throttling so much that it under-performs when compared to last generation i7.
You will probably be right to blame Intel and their failure to innovate. Their last 3-4 generation of CPU are just iterations of the old design with almost non-existing performance improvements but better efficiency. The desktop version of the i9 was running so hot that I gave up buying an new a PC. I didn't want to buy a CPU that required the best cooling you can buy just to run it without any overclocking. I continued to do all my work for almost a year on a mid 2014 Macbook Pro with a 4th generation i7-4980HQ CPU. This CPU is so good that any upgrade to the new Macbook seemed a complete waste of money when you consider the performance gains.
To add insult to the injury in Apple's case with the Macbook Pro, the problems seems to be the voltage regulator, not even the CPU getting hot. Watch this video from Louis Rossmann to understand why this is bad. The original post on this also comes instructions how to work around this issue, if you happen to own one: Optimal CPU Tuning settings for i9 MBP to stop VRM throttling / Explanation of Apple's Engineering Failure.
In Dell's case, you can apparently squeeze more performance by undervolting. This means that Dell did it's part when it comes to power delivery for it, and the throttling issue is only caused by i9 thermal issues and the laptop's old thermal design. You can find more about tweaking the XPS on notebookcheck.net: Dell XPS 15 9570: 15 % more performance by undervolting.
For the next 3-4 year I will probably not upgrade my XPS, but I hope that next time AMD will be more competitive in the laptop market and software companies will optimize for AMD as well.
Later update: Apple released a software update to "fix" the issue. Behind the PR talk, you can see that what they did is limiting the boost/peak performance to keep those power chips cool. I guess less performance but sustainable is better than no performance at all. I wonder why people would buy the i9 in this conditions and not the i7.