Etrecheck says performance is poor. Any way to find out whats wrong?
Etrecheck is usually pretty reliable.

Your iMac is running on one of the latest software versions on a 10 year-old machine. 

Your iMac is too old to handle the latest software and to support peak performance therefore Etrecheck is displaying that message.

If you can I recommend downgrading your OS to an older version, one that your iMac can handle better. You can see how to downgrade in this article. 

If you don’t want to downgrade I still recommend creating a backup for your files. 

An additional test you can run is an Apple Hardware Test, by holding the D key on startup which might detect any other issues.