Hello DeeEes,
Thank you for using Apple Support Communities! I understand from the information you provided that your iPhone is connected to Wi-Fi but you have no or slow internet; I'd like to help.
While the following resource is for if your iPhone won’t connect to a Wi-Fi network, it provides some relevant troubleshooting steps to try for this issue:
If your iPhone, iPad, or iPod touch won’t connect to a Wi-Fi network
In addition to these steps, make sure you have the latest version of iOS installed. Currently iOS 13.1 is the latest version. You can see how this done here:
Update your iPhone, iPad, or iPod touch
Hope this helps.
Best Regards.