Is High Sierra noticeable slower than Sierra?
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.