WiFi slowness could be due to a variety of factors (WiFi access point type, settings, useage from other WiFi devices, protocols supported...). I put my 6+ in Airplane mode with WiFi on and ran Ookla Speedtest. 33.25 Mbps download and 7.84Mbps upload with 385ms ping latency (from farthest point in the house away from the router).
Since the Speedtest talks to an external server infrastructure and network (your cell provider, internet, hosting server provider) that may have its own variability, the results received from one test may not be the same for the next, but in general if three tests were run with the same variables each time; then the results should be roughly the same.
My router is the current Airport Extreme generation which supports 802.11ac. To me WiFi seems fine at home, but that's mostly where I use it.