I've got a core i7 6 core 3.33GHZ system with 24 GB of memory and a 2TB hybrid drive running windows 10. It runs well and is protected with Norton security, Malwarebytes, and superantispyware.
It's been running very well for at about 3 years. A few days ago they system went into slow motion.
It now takes forever to boot and when it does I can't really be productive. Naturally I ran all my security scans but they didn't change anything. However, when I unplug the ethernet cable the system returns to normal or at least close to normal.

Any ideas of what may be going on and how to resolve it?