1. All systems have Full Premium, except Mail Shield and Real Site, with SecureLine VPN. I have never logged in to VPN on any of my machines. I don't have Cleanup nor Tracker installed anywhere - they were VERY problematic.
2. I just fully uninstalled all Avast from the laptop, ran Ookla SpeedTest app, got 940+ Gbps download. Installed Avast minimum configuration - File, Behavior, Web. SpeedTest numbers ranged from 117 to 816, mostly at the lower end. Uninstalled and again verified 940+. Installed Avast File Shield only. SpeedTest numbers were just as bad. Also, I verified this is a real slowdown by comparing some huge ordinary web downloads using different machines.
3. Only true improvement comes from complete uninstall.
All this makes me suspect something in your core files for even minimal installs interacts with web access (not lan access, though). Could it be because the laptop has both wired and wireless connections, even though the wired connection is the only one active? Another difference I didn't initially mention is the laptop is an i7, with Intel chipsets. My other computers are all AMD. However, all network adapters are Realtek. The laptop has a USB adapter, but I tried it on other machines, and they are still fast. Could it be some silly Dell software that I can't diagnose? I removed that stupid SmartByte app long ago... I tried setting the laptop to fixed IP address with no change. Could this be an IPv6 issue? I have never bothered to learn anything about IPv6, but I see addresses reported when I run ipconfig.exe.