I'm having very bad LTE speeds on Sprint as well (Yes, Sprint is bad, but my testing is in an area with great speeds, within eyesight of the tower). My 5 was fantastic - 20 Mbps down, 6-8 Mbps upload average easily.
My 6+? Even when connected to the same tower at the same time, I'm lucky to get half the download speeds. The HIGHEST upload speeds I have seen is 1.5 Mbps, but it is almost always very close to 1 Mbps.
Sprint chat support told me that it was a known issue - that both Sprint and Apple were aware of - and a fix, from Apple, would come soon.
Another Sprint rep recommended I return the phone. But they've made it absolutely clear its not an issue with my SIM or account. I can swap my SIM into my iPhone 5 and get my normal LTE speeds again immediately.
I really am unsure of what to do.
By the way, I'm in the San Diego area, and know firsthand of other people with the same exact issues with their iPhone 6's.