I missed that thread...
OK to make things clear, I would have to uninstall avast completely to see how the behavior is. What I've noticed so far, with Avast5 installed, is no slow down of my bandwidth, but high CPU load on system/NT Kernel and System, and WmiPrvSE.exe (WMI provider host). The faster the download is, the higher the CPU load is. And I'm damned sure I never observed this when I was running Avast 4. Downloads use to run in the background without anything to notice.
It's too late to check how things would be without Avast, I'll test that tomorrow.
edit: avast processes don't show any CPU load during these downloads. Turning the web shield off doesn't change anything with the CPU load, if attempting to restart the download. I tested that with a Linux ISO. I guess the webshield, when turned on, scans some of it, as all packers are enabled for this shield, but it's unnoticeable.