Without using a download manager, I typically can get around 25KB/sec downloads (on DSL, PPPoE), which I suspect is my provider's limit. If I'm downloading something large enough to make a dlm useful (I like Free Download Manager) and I've got 12 sections running, I can get 300+ KB/sec if the server at the other end is reasonably good.
I've only seen avast slow internet activity under two fairly specific sets of circumstances. If it's doing a check for updates, and traffic is heavy, it might take a while to work its way through the list of avast servers to find one it can connect to and download from.
And the other possibility, which I only saw explained in a fairly recent thread here: any kind of right-click context-menu activity, especially if a number of installed apps use the context menu, means avast has to check each such application before you can make a choice, typically several seconds.
My local independent ISP has a generous 50 GB/mo. cap, of which I'm typically using 3-5 gigs (I'm not into video streaming or similar heavy usage). A couple of days ago I switched to a wireless router so my wife
could also have internet access, plus my son hooks into that with his laptop, but I can't see using more than double what I am now.
Sorry for the rambling, but hopefully you can pick some useful info out of it.