MacBook Pro 2015 really slow since macOS Sierra / High Sierra [With maybe a start for an answer] Since macOS Sierra or High Sierra, macOS has become really, really slow.
Reinstalling El Capitan (even today) solves the issue.
GNU/Linux and Windows are also much faster, more like El Capitan was.

By slower, I mean twice (or more) the time to build a simple C project (from 5-6 seconds to 15 seconds), the whole system is really sluggish, I really think that it is a macOS issue.

For information, I tested mainly with gcc-9 and clang-10 with my school projects.
I also used tcc 0.9.27, built from source, with a dummy make+C project, see below.

I have already tried to reinstall macOS.

I am not sure about what to do, have anybody ecountered this ?

I need macOS for the iOS/tvOS toolchain, therefore I cannot switch.
Switching to a GNU/Linux distro would be nice if I could.

If anybody needs more specific data, do not hesitate.
Many thanks for your help !



Edit: I tried to replace a kext and got significant improvements, see below.



Things I tested always on the same computer with this dummy repo edrflt/build_speed :

tcc 0.9.27, built from source make -j5

Ubuntu 20.04 (fast)               0,94s user 0,53s system 342% cpu 0,428 total

Mac OS X Yosemite 10.10 (fast)    1,84s user 1,05s system 284% cpu 1,016 total

Mac OS X El Capitan 10.11 (fast)  3,04s user 1,18s system 316% cpu 1,332 total

macOS Sierra 10.12                4,70s user 2,64s system 265% cpu 2,769 total

macOS High Sierra 10.13           6,03s user 2,79s system 257% cpu 3,428 total

macOS Mojave 10.14 (slow)         8,30s user 3,09s system 231% cpu 4,912 total

macOS Catalina 10.15 (slow)       6,01s user 3,35s system 193% cpu 4,827 total


Build time is multiplied by almost 5 vs Ubuntu 20.04 which takes less than one second.

gcc-9

Ubuntu 20.04 - gcc 9.3.0 from apt

make -j5  10,15s user 4,69s system 361% cpu 4,111 total
make -j5  10,13s user 4,74s system 357% cpu 4,162 total
make -j5  10,27s user 4,54s system 359% cpu 4,119 total


macOS Catalina 10.15 - gcc 9.3.0 from brew

make -j5  13,95s user 10,55s system 298% cpu 8,204 total
make -j5  16,91s user 12,81s system 252% cpu 11,752 total
make -j5  14,05s user 10,81s system 295% cpu 8,412 total




Kext replacement - significant improvements

After some investigations, I found that IOPlatformPluginFamily.kext could be the culprit (or one of them).
I replaced the kext in my setup of Catalina with the one from El Capitan, ran the same tests, got these results (with tcc) :

make -j5   5.75s user 2.21s system 278% cpu 2.862 total
make -j5   5.78s user 2.13s system 278% cpu 2.837 total
make -j5   5.74s user 2.13s system 277% cpu 2.836 total
make -j5   5.74s user 2.14s system 288% cpu 2.736 total


What I understand here is that I got performances around the one I could get with Sierra. I am not really sure about the reasons.
I could see that Mac-E43C1C25D4880AD6.plist from X86PlatformPlugin.kext inside IOPlatformPluginFamily.kext changed (this file is related to my Mac model) but I'm unsure about this one.
I also saw that the binary X86PlatformPlugin changed but I did not any further investigation on it.
I will share any additional data I can find.



Notes :

My MacBook Pro is this one :
MacBook Pro (Retina 13, early 2015) - MacBookPro12,1
i5 5257U - 8GB

I am totally open to any tests, any suggestion.
I know I don't know enough about low level OS components and compilers, therefore if you have the time, explain me like I'm five.
I will take any time that would be needed.
This is not at all about macOS is better, GNU/Linux is better ... no no, this is about "I want my computer to get his speed back".
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