Finder

Also download and run Etrecheck. Copy and paste the results into your reply. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance. This will help tell us if there's any 3rd party software that might be contributing to the problem.

Create a fresh user account and test.

macOS Sierra: Set up users, guests, and groups on your Mac

Test in Safe Mode:

Use safe mode to isolate issues with your Mac - Apple Support