Yes, the 'bulk' option is very often the problem solver. At least that fixes the error for me every time it presents, just altern between checking and unchecking those options until its back to your regular download speed. If the error presents again try checking those options again. Also, we recommend that you start the troubleshooting by power cycling your router and then moving on to further solutions.Necroposting but I think I found the solution for people that only uses windows anti-virus, uncheck Bulk download and Agressive download options in the launcher. We hope you get to fix the Steam Deck slow Wi-Fi. If you cannot get rid of the Steam Deck slow Wi-Fi on or downloading after re-imaging, force rebooting, you should contact Steam Deck and ask them to perform the necessary repairs on the console.
If you are still having the Steam Deck downloading slowly then move on to the last solution.
If you have a Windows PC you need to use Rufus utility and select the recovery file and write it to your USB drive.You need to have a USB with 8GB minimum space and download the recovery image from the Steam store in the USB before starting the solution.
This system image is not officially launed for the SteamOS 3 version so, there are chances it might not work properly but according to Steam it will be launched soon officially.