Several fixes may be able to resolve this issue, i had this problem once and my first fix here fixed it for me, but might be different for you. Before you start just turn on and off your router, if nothing changed feel free to try some of this i found here.
Change Download Region:
Steam allows you to change your download region, which can sometimes improve download speeds. Go to Steam > Settings > Downloads > Download Region, and try selecting a different region that's close to you.
Restart Steam:
Sometimes simply restarting the Steam client can help resolve download issues like yours here.
Disable Windows Metered Connection:
To disable this follow these steps, if this doesnt resolve your issue go ahead and enable it again if you want to.
- Right-click on the Windows Start button and go to Settings.
- In the left-sidebar, select Network & internet.
- Click on Wi-Fi.
- Click on your Wi-Fi connection.
- Turn off the toggle next to Metered connection.
Disable firewall/antivirus:
Sometimes firewall or antivirus software can interfere with Steam downloads. Try temporarily disabling them to see if it improves your download speed.
Don’t really have much else for you, but take a speedtest and check if you get the speeds that you regularly have, you can also close any unnecessary applications while you're downloading from Steam, as other applications might take up the network.