There are several possible explanations. For comparison, on my iPhone 7 Plus I just got 114 Mbps on my 100 Mbps connection on the 5 GHz AC WiFi band, but only 45 Mbps on a 2.6 Ghz a/b/g/n band. So what is your WiFi network configuration? How far are you from the router (this affects the retry rate)? What other devices on your network are consuming bandwidth?
Also, I'm not sure of the network limitations of an iPhone 6+. You may be running up against the limits of the slower model 6 Plus processor.