Several users have reported that they are experiencing slow Wi-Fi after they updated their iOS software. If your device is having poor Wi-Fi performance problems, and thus internet connection is very slow, try the following tips to resolve your issue:
Check your router
This problem may be due to your router problems. Turn off your router and switch it back on. Check to see if there is an update for your router’s firmware. Try temporarily disabling your router’s security settings to see and rule out any encryption issues that may be causing the problems. See this article for recommended settings for Wi-Fi routers and access points. Several users have reported that disabling QoS (Quality of Service) on their router speeds up their Wi-Fi connection. Please refer to your router’s manual for this.
Forget this network
Try resetting your Wi-Fi settings by tapping Settings > Wi-Fi and tap blue arrow icon of your Wi-Fi connection > and tap “Forget this Network”
Turn off LTE
Tap Settings >General >Cellular and toggle Enable LTE to Off.
Reset your network settings
To do this, simply tap Settings > General > Reset > Reset Network Settings. Note that This WILL remove any Wi-Fi passwords and VPN, and APN settings that your device saved.
Restart your device
Simply restarting your device may solve your problem. To do so, simply press and hold the On/Off button until “slide to power off” appears and then slide the slider.
Try changing your DNS settings to 184.108.40.206 which is Google’s public DNS. To do that, tap Settings > Wi-Fi > find your Wi-Fi connection > tap the blue arrow > Tap where the DNS numbers are and change it to 220.127.116.11. See this article for more info.