WiFi has been crap on my s4 since day one. Constantly drops or slows significantly on N or AC or G networks. I tried 3 routers and I have dozens of other devices with no problems. Countless times a day I have to toggle WiFi on and off to restore speed for another 5,10 or 15 minutes. YouTube is the worst. It just hangs when the phone drops to 1-3 Mbps down from 30. This happened on stock and all stock like ROMs. WiFi is this phones tragic flaw. The problem gets no attention online because people think it only pertains to dlink routers which is completely untrue.
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