Greetings WillHammond,
Having a slow connection can be very aggravating, and we’re glad to help!
How to back up your iPhone, iPad, and iPod touch - Apple Support -- We know you created a backup before you updated, but if you’ve been using your iPhone since, we recommend backing up again before proceeding.
Return iPhone settings to their defaults - Apple Support -- Please review the information under “Reset Network Settings” to learn what this is is going to do. You may want to write down any passwords to the Wi-Fi connections on your iPhone if needed. Once you’ve review the information, go to Settings > General > Transfer or Reset iPhone > Reset > Reset Network Settings.
Once your iPhone is back to the Home screen, see if your Wi-Fi connection has improved.
We also want to make you aware that there’s no Apple supported method to downgrade the iOS installed on your iPhone. This is what an iCloud or computer backup includes: Backup methods for iPhone, iPad, and iPod touch - Apple Support
Please let us know the results when you're able.
All the best.