One of my online friends downloaded a few big files the other day, about 3GB total, and his internet has been slow and unreliable ever since. (disconnections and major slowdown)
He has Comcast cable internet. They have a "cap" of 250GB a month, so he can't be anywhere near reaching that limit. He doesn't download big stuff like this often.
He has a Mac.
He has a modem (of course), a router (I think), and some kind of hardware firewall device.
He tried unplugging them all for a bit, but that didn't fix anything. I might ask him to try disconnecting the firewall and maybe router, but I don't yet know if that will work.