Slow Samsung S8 Oreo. Solved!

Updated, since I found the issue.

Suddenly my Samsung S8 Android Oreo become un-responsive and very slow. 

Screen button was tactile but power button did not work, unless presses for very long time.. 

When it decided to unlock the screen it sometime paused or froze using the apps and browsing.

I installed a CPU monitor (Tinycore) and noticed that the cores 5,6,7,8 never got above 741Mhz. 

First I tried to start the phone in fail safe mode and it did help! So then we can rule out the phones own hardware. So the suggestion was to uninstall the app causing this. How can you find that out without a working CPU monitor / app in Oreo. Impossible, unless rooted.

I uninstalled a lot of apps to no help.

I wiped the cache partion and immediately after I wiped the cache partitions the second half of the cores could use all their power.

I did it twice, but it has not completely solved the issue.Just that when the phone gets slow it has more CPU capacity to not go totally slow.

Wipe cache partition

1. Turn off the device.
2. Press and hold the Volume Up key and the Bixby key, then press and hold the Power key.
3. When the Android logo displays, release all keys ('Installing system update' will show for about 30 - 60 seconds before showing the Android system recovery menu options).
4. Press the Volume down key several times to highlight “wipe cache partition.”
5. Press the Power key to select.
6. Press the Volume down key to highlight “yes” and press the Power key to select.
7. When the wipe cache partition is complete, “Reboot system now” is highlighted.
8. Press the Power key to restart the device.

Solution

Then I got to think that maybe it's the SD card. I checked the My Files -> and there it said SD card Not inserted.
So something has happened with the SD card (possible Max write cycles) so I opened the sim card / memory card holder and removed the SD card and since then it has been working perfectly again.

Plugging the SD card into my computer still shows the content on it so it might have been just bad contact also in this case.