"Anyone experiencing really slow WiFi on their new iPhone 6 Plus? Mine is super slow. When I switch off WiFi, LTE works perfectly and very snappy. I didn't have any issues with my 5s and not having issues with my iPad Air. I doubt it's an iOS 8 issue because the 5s and iPad Air are running iOS 8. WiFi also works fine on my 13" MacBook Pro Retina. It's not a router issue. Any ideas??" asked by Mikef91 on the Apple's office discussions.
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Greyed-out, slow or no Wi-Fi signal at all are among the top three biggest Wi-Fi headaches. Even though the iPhone 6 and iPhone 6 Plus came with iOS 8, Wi-Fi problems are still not completely avoidable. Before searching for solutions to Wi-Fi related problems, always update your device to the newest version of iOS. If that doesn't work, then it is time to dig deeper.
1. Reset Network Settings
Settings > General > Reset > Reset Network Settings.
This is a solution suggested by Apple Service. Resetting your network settings including Bluetooth pairing records, Wi-Fi passwords, VPNs, and APN settings; which flushes cache and clears DHCP settings and other networking related information.
2. Disable Wifi Network Services
Settings > Privacy > Location Services > System Services.
This only relates to your location being used for network and not the Wi-Fi itself.
3. Switch DNS To Google
Often times, if your ISPs DNS servers are having problems, you can switch to Google DNS by going to Settings > Wi-Fi then tap the "i" icon next to your router, scroll down to DNS and tap the numbers, then enter "18.104.22.168". Restart your device and check if it works.
4. Change The Routers Broadcasting Band
Change the routers Wi-Fi broadcast from an N network to a G network, or from 2.4GHz to 5GHz - this depends on your individual router hardware.
5. Factory Reset
Settings > General > Reset > Erase all Content and Settings > Erase iPhone.
Last but not least, restore your iPhone and set it up as a new device.
Other tricks like placing the iPhone 6/6 Plus in a refrigerator, freezer it for 15 minutes, or the opposite using a hair dryer to heat up the device might also be of help.
However, if all these software related tricks don't help, it could possibly be a hardware problem with the Wi-Fi antenna. Then the only fix is to replace your antenna with a new Wi-Fi antenna replacement.
Here are links to the parts and tutorial guides' for reference.