I had a similar experience with my Galaxy Tab 3. -20db worse reception on WiFi compared to my other devices. It also slowed down to 5Mb/down after some usage that a reboot would fix. Then the power button got sticky so back to Sprint it goes. I didn't realize how important a good WiFi connection is because I don't use WiFi on my T-Mobile phones since they have unlimited data.
Castrol EDGE 5W-30 is my drug of choice.
Primary: iPhone 6 64GB, Lumia 635, & LG L90: 5 lines, 1x $15 5GB data, 2x $5 Android web, 1x $2.99 T-Zones, corp discount + $10/mo loyalty discount = $93/mo tax inclusive
Secondary: iPhone 4S & Optimus Dynamic II: TracFone (Verizon); LG Mach: RingPlus (Sprint)
Tablet: iPad Mini Retina w/ T-Mobile 4G LTE