Why Macbook started to slow down (Solved)?

Question

macOS

Hello.
I will try to describe the problem in as much detail as possible.
I have a MacBook Air 13 -- early 2015, A 1466
Specifications: Core i5 1.6, 8GB RAM, 256 SSD, battery has only 150 charge cycles.
C recently started to slow down a lot. It takes longer to load than usual, loading in safe mode can generally take about 10 minutes. Immediately after loading it works fine, but after launching several applications it starts to slow down terribly. This greatly reduces the speed of the SSD. I checked the speed through the Blackmagic Disk Speed ​​Test: immediately after the system boots up - writing / reading is almost the same 1000-1100 MB / s, and when it starts to slow down - it drops to 450-500 MB / s. At the same time, everything that is possible begins to blunt: a video on YouTube, switching between desktops and browser tabs, a code editor, launching applications, and even just typing in the terminal. It doesn't matter - 3 applications open or 10 - it works equally slowly. But if you don't do anything for a while, then the system will return to normal operation and the SSD test will again show 1000-1100 MB / s, but happiness will end again if you start a new video in YouTube, or open another file in the editor. The decrease in performance is especially noticeable when opening a large file in VS Code, for example, code highlighting in a file 10,000 lines long lasts 2-3 minutes, and after that the system starts to slow down wildly again.
I ran various utilities to check the system and SSD - Apple Diagnostics, DriveDX, Disk Drill, TechTool Pro, EtreCheck - only EtreCheck showed "Low performance", but otherwise the system passes all tests. There are no heavy processes in the System monitor, "kernel panic" is not observed, the system has been updated to the latest version.
Mojave is currently installed, but I tried installing both Sierra and El Capitan - it did not help. Fully formatted the SSD - to no avail. Spotlight has fully indexed the system. Booted from an external drive, but the external drive is an HDD, so the system runs even slower on it.
I read somewhere that a faulty wi-fi module can affect the operation of the SSD in such a way, and if you turn off wi-fi, then everything works as it should - in my case it did not help.
What could be the reason?
Maybe someone faced such a problem?
UPD: The motherboard became the cause of the problem. The service center said that some components on the board burned out, and because of this, the Mac misunderstood the battery (this is in general terms - the master spoke in some terms). Repair cost $100.

Answer 1

256 SSD

How much free space do you have there?
Well, it would be nice to look at the SMART disk.

Answer 2

1. reset SMC and NVRAM.
2. Cleaning from dust.
Did you?
Considering that you have already performed enough different manipulations, it is best to go to the SC without delaying it. It may be that one incorrectly working node will cause the failure of another!
It can be anything - an imperceptible fall, liquid ingress, or just a short circuit occurred and some element fails. It often happens that a client brings a non-working laptop, I ask, “Did you upload it? - Not!" and traces of liquid inside. People are sincerely surprised that they did not notice how they flooded or someone flooded and kept silent.
Wifi turned off visually?

Answer 3

trigiw And what service did you contact? Can you recommend?