Killer E2500 Gigabit Ethernet Issues | DELL Technologies
The generic fix is...
Click Start>Run>services.msc and click OK. When services.msc opens, find each Killer service (and xTend services, if any listed), Double-click each of them, one at a time, to open it. Set the Startup Type for every Killer (and xTend) service to disabled.
Exit service.msc when all Killer (xTend) are disabled, reboot PC, and check ethernet performance again.