Is High Sierra noticeable slower than Sierra? I I have a macbook pro 2012 15" i7 (quad core) with a SSD drive and 16G RAM. Last night I've updated from Sierra to High Sierra and I've noticed that everything seems to be kind of slow. Before the upgrade everything run snappier than now on High Sierra. I just upgraded from the mac app store and not a fresh install of it. 

Should I run a fresh install in order to get a better performance?
No - High Sierra isn’t generally slower than Sierra so there’s likely something else going on.

I like your suggestion to run a fresh install. That’s almost always the fix when a system gets slower and slower and even a fresh install of the same OS as you have running for 6 months to 2 years straight often makes everything snappy and you happy again.

I’d do one last backup and erase install High Sierra and migrate the data back. Take some timings so you know objectively how long a boot takes and how long the things you think are repeatable (opening 5 tabs in Safari / opening a large spreadsheet / etc...) so you can then dig in if the reinstall doesn’t make everything better both subjectively and objectively.