@Finder1
Go to our official website at: www.tp-link.com, then navigate to the Deco M5 support page (search for Deco m5, and select support when getting the result), go to the firmware section.
First select your hardware version, using the wrong hardware version may brick your Deco.
Download the firmware before the April released one. Extract the downloaded file and you will be left with a folder containing a .bin file. Now log into the Deco using a computer web browser. Here is how you would do that: https://www.tp-link.com/us/support/faq/2641/
Go to Advanced > System > Firmware Upgrade, and upload the .bin file.

This should downgrade the mesh network back to previous firmware. Test the speed to verify if they have returned, if so, please go back to our website, and now download the latest firmware there. Make sure you download the firmware for your hardware version.