MacBook Pro 2015 really slow since macOS Sierra / High Sierra [With maybe a start for an answer]
I have exactly the same issue than you:
First, I noticed that the entire system was being slower an slower after every upgrade. It could be because of the new improvements or the patched Meltdown and Spectre.

But, then I did my own research:
I installed GNU/Linux, and saw the same improvements as you: more responsive system, faster compilation. But, okay, GNU/Linux is known to be faster, but it's not normal to have such a big difference. And these are also patched for meltdown and Spectre.

Next step, installing El Capitan:
I remembered how macOS was faster in the past, so I decided to give it a try and install El Capitan on my Macbook, And yes, EVERYTHING was faster (and you need to play around to get it installed because apple has probably &quot;forgotten to renew the certificates&quot;).

Now that I'm sure that there is a thing:
I talked about it to see if I'm the only one experiencing the issue.
I have a friend with the same Macbook as me. So we set up a Genius bar meeting. We went to an Apple store, showed them the facts, one Macbook on El Capitan, the other one on Sierra (yes it's a long time ago)
They saw we were right, and the Genius said: &quot;I'm sorry, I see there is a problem, but I can do nothing for you, maybe try sending a Feedback&quot;
We went out with no hope. But we sent a feedback, which was ignored by Apple.

The last thing, is more of a rumor, but I'm taking it into consideration because it explains why:
I spoke about my Macbook problem to my colleagues. And one of them, knew there is a issue on these Macbooks, so I asked him more details, he has a friend working as a developer for Apple, they are totally aware of the issue with those Macbooks, but the instructions are clear: &quot;Don,'t talk about it, forget it&quot;
Technical details:
It's apparently a kext issue with the SMC (or the power supply controller?), they used a cheap controller. This controller's job is to provide sufficient amount of power to the CPU (It's like a feature for improving turbo boost I think)
And his work is not done properly which is why we are experiencing these issues.
The controller is disabled on other operating systems because they don't have the kext. That's why we only have the issue on macOS
That's a pretty convincing reason: they blew the driver and don't want to repair their own mistake, so we have to buy a new one pretending our laptop is old. But there are only 5 generation of Intel processors between this Mac, and today. And we all know how minimal the improvements are.

If anyone has an idea that could make Apple/(more) people hear us, or find more proofs that Apple is duping us, please let us know