Since you've already confirmed its a Cat6 cable, I'd try 2 things:
Change your DNS settings. Almost anything is better than ISP default. I'd try using 8.8.8.8,8.8.4.4 as a baseline to use the Google/Akamai DNS lookup servers. Possibly 1.1.1.1 if you'd like to try a newer optimized service from Cloudflare. Of course this shouldn't affect throughput, really, but its something to check.
Check if the ISP supplied hardware creates a guest network of any kind. This is default behavior for Comcast (granted, that's not your ISP) and may be for other providers as well. While its unlikely to cause such a strong impact, reducing guest wifi networks may help.
Other than that, if you consistently get high speeds on one computer using the exact same hardware for internet connection, and another computer is consistently slower, its possible you may have malware or other nasties on your slower machine. Might try Malwarebytes Anti-Malware in safe mode boot to make sure you're not part of a botnet.