Great if it's passing 100 MBPS than Duplexing is not the issue.    1 Look for a program Called SmartByte in Start Menu (uninstall it)  Reboot  Check in Run > MSCONFIG > Services : If smartbyte is not listed, run the speed test.    2. In Task manager > Startup > Disable all apps  Reboot and run the test again, if results are fine enable these apps one by one.    Last option prior contacting Tech Support for Motherboard replacement (Jack is on MOBO) is to reinstall OS.  And when it comes to OS reinstall, do not RESET or use factory image  Perform a clean install using > F12 > SupportAssist OS recovery > RESET > RESET and update
I have 2 Inspiron 660's desktops. I have Spectrum internet and on one of the computers using speed test, I get 230 ish download and 10 upload which is what spectrum says I should get but on the other computer I only get half the download speed and about the same upload. Both are hard wired to satellite mesh routers and I have swapped the computers to make sure it wasn't an internet issue. Does this sound like a hardware issue. Thanks for the help. 
LAN speeds for 660's