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, Galaxy Note 3, Galaxy Avant: 5 lines, 3x $5 Android web, 1x $2.99 T-Zones
Secondary: iPhone 5 & Motorola E LTE: TracFone (Verizon)
Mobile Broadband: ZTE Z915 LTE hotspot (5GB for $10) + iPad Mini Retina w/ T-Mobile LTE