Slow High Sierra after restore from Time Capsule
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!