Searching forums only give you those who HAVE problems. And likely its due to their own issue or hardware issue.
There is NO reason for an older Samsung Galaxy phone to slow down if you have done the following.
1. Replaced your battery
2. Updated your OS
3. Keep it clean (no viruses and capacity issues)
4. No hardware issues.
NOTE 8 had battery issues (FIRE)
Why do you want software updates anyways?? What can your phone not do when compares to the phones a year newer? Its pretty much a wash, OS updates should make your phone more efficient. End of story.
Note 7 had battery issues....not note 8