Network speeds on computer ran via CAT6 are DEVASTATINGLY low [migrated]

Solution 1:

You seriously need to reduce complexity to track down such a problem. Judging from your description, you're currently troubleshooting this setup:

Spectrum router <-> Netgear R6400v2 <-> Powerline Adapter <-> Powerline Adapter <-> Laptop

with the suspected Cat-6 cable in between the Powerline Adapter & Laptop (or the routers, or Netgear & Powerline, doesn't matter). Unless that cable is damaged or DIY you can most likely rule it out.

Instead of that long chain, connect your laptop to the Spectrum router. If that's bad call Spectrum (or maybe test another, known good cable). If it is OK, test behind the Netgear next. And so on.

My money is on the Powerline run.