Why is my WiFi so messed up?

My Samsung Galaxy s4 Active's WiFi has been acting up since I got it. It randomly drops the signal and only works for about 30 seconds at a time, and when it does work its very slow, i'm talking like 500kb per second....

Most of the time it shows that i have full 4 bars, but doesn't work. One time i waited for an app to load for about 5 minutes, only for it to say "Network connection lost"

I have tried almost everything,(including static IP,Power saving Options,Configuring Routers, Updating Firmware, Rooting the device, and several WiFi fixer apps!)I even went as far as factory resetting it twice

We have 4 other Samsung phones in the household but none of them have this problem, nor do the computers or gaming consoles. My playstation gets a good 25mbs, and most phones around 15mbs, but this one not even 1mbs.

Sounds like hardware issues. Likely damaged/weak connection to WiFi antenna. If I remember correctly S4 series antennas are in the battery door. Check the contact prongs One maybe bent. My Note 2 had the same issue, after 18 months I had to replace battery door. Bent to many times until it broke off.