So I had a hard drive (SSD) failure some time ago (it lasted 5 years of intense use, so I kinda expected that to happen).
Luckily I had my Time Capsule running all the time, so it was a no brainer to buy a new SSD for my Macbook Air and restore it.
However after restore I found out that my laptop wasn't quite as fast as it used to be, moreover I'd get frequent freezes & beach-balling that would last up to 20 seconds. Needless to say, I was quite concerned – I was running many system diagnostics, tried cleaning it up, did SMC & PRAM reset, but nothing seemed to help much.
What was the issue?
Best Answer
It turned out that my system restored in the old "MacOS Extended" file system. You can check yours by right-clicking your hard drive in Finder sidebar pane and choosing "Get Info". The correct entry for High Sierra should be APFS, like on the picture below:
So in case your restore left you with an older file system, you need to reinstall MacOS from the App Store. Unluckily I don't remember the exact details how I made the installer go through the installation again, but I managed to do this by finding the installer in Applications folder - without starting from the recovery partition (which would probably try to clean install my OS). I hope that helps someone!