Downgrade iOS

Is this true for all Wi-Fi networks, or just your Home network? If it’s just your home network do the following:

• Restart your router by removing all power for about 30 seconds
• Restart your phone after the router is back up and working—>Restart your iPhone - Apple Support
• Test using the speedtest.net app or website
• If it is still slow go to Settings/Wi-Fi, tap on the i next to the network name and Forget This Network, then connect to it again
• If it is still slow go to Settings/general/reset - Reset Network Settings

One of these steps will fix it.

I see that your phone is in low power mode. The whole phone is slowed down in low power mode. So try turning that off first, then follow the instructions that I have already posted. For comparison, this is my phone with iOS 13.3.1:

Then your phone is broken. Contact Apple support to get it fixed. It has nothing to do with the iOS version.

No. It is not possible to run an older version of iOS on your iPhone.

Sorry, downgrading is not possible.