Windows 10 updates are released each month to fix security flaws and address quality issues. This process should run seamlessly in the background but can cause problems on a slow internet connection.
The Windows 10 May 2020 update (build 2004) added new options to let you take control of the update system. You can now set hard bandwidth limits which Windows Update promises to respect, ensuring your internet doesn't slow to a crawl while updates download.
Open the Settings app (Win+I keyboard shortcut) to begin. Click the "Update & Security" tile from the homepage. Use the menu on the left to switch to the "Delivery Optimisation" page.
While the naming may not be intuitive, "Delivery Optimisation" refers to the mechanisms by which Windows Update acquires new software. Click the "Advanced options" link at the bottom of the page to impose bandwidth restrictions.
You can choose between setting an absolute bandwidth cap or using a dynamic cap as a percentage of the available bandwidth. Prior to the May 2020 update, only the latter option was available.
To set a hard cap, select the "Absolute bandwidth" radio button and then use the two checkboxes and text inputs to adjust the background and foreground bandwidth caps. It's a good idea to use a lower value for background usage - this ensures automatic updates don't sap your bandwidth, while enabling manual "check for updates" presses to run at full speed.
When using an absolute cap, you'll need to work out which values are best for your internet connection. Try running an online speedtest to check your line's maximum bandwidth if you're not sure what to use. Alternatively, use the percentage-based system to have the cap scale with your available bandwidth.