You may be able to fix a slow internet connection by resetting or reconfiguring your iPhone’s network settings, or speeding up your browser app. Otherwise, the problem is likely due to your Wi-Fi — take steps to boost your signal strength or fix your router if it’s not working.
Another reason your internet may be slow on your iPhone is that your service provider may be throttling your connection. Preventing ISP throttling is just one of the reasons why using a VPN is a good idea.