Well try
https://www.speedtest.net/

if that work then its from your ISP into steam there is a isssue and that is your ISP problem for not can deliver it, but many forget here ISP has only promish in own ISP that speed not rest of the world.

ps.
Change steam DL zone and hope that angle or path or route is better for you into a steam server, this will not work if own ISP is the bottleneck.

and yes steam summer sale event is over now its STEAM Automation Fest, not sure how much traffic that generete. among all our ISP , and how many that buy And mass DL.
still a event yes it is. but how many steam user that go shop amoke i do not know.