If you just upgraded to 10.12.6, wait at least a few hours with the computer operating and turn the computer of and on again and see if that makes a difference. If you went from a previous version (El Capitan, etc.) to Sierra, post that information.
If this doesn't help, run in Safe Mode and see if that makes a difference.
https://support.apple.com/en-us/HT201262
If not, use the Activity Monitor app to see if your CPU, memory or disk is being taxed (it sounds like you may have already done this).
https://support.apple.com/en-us/HT201464