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 into 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.

What happend from your ISP network into other ISP partner's is also your ISP problem, you are not customer there, and as gamers here we are tirid of all ISP is full of crap and live in stoneage, with past Solution if they cant figureout have Main line free to our game client then they sux at it. ( we are talk ing about common traffic pattern here, not even game server lag issue. ) make sure you pc is not busy at 100% in drives write issue, we cant fix DL then you tell server you can't keep up. ( thats is a pc issue and you are the bottleneck. )

Use windows resource meter to notice own hard drives and other 100% in use bottleneck. rest is seen as own ISP issue with there partners partner.

ps.
Why we doubt's it's a steam issue they handle themself very well doing Corona.