Hey there fondazion,
It sounds like the slow performance may be isolated to Wi-Fi. Have you had a chance to test another network? If not, let's start there. If the issue only happens on your network, restart your router and test. If the behavior continues, contact your service provider.
For general troubleshooting of Wi-Fi on your iPhone, let's follow these steps: If your iPhone, iPad, or iPod touch won't connect to a Wi-Fi network - Apple Support
It may help to explore other reasons the iPhone may be performing slower than expected. Check out this guide with some information to look over: About iOS or iPadOS performance on your iPhone, iPad, or iPod touch - Apple Support
Take care.