Make sure you're on the 5Ghz network, as a previous poster said, sometimes routers name the 5Ghz and 2.4Ghz networks the same, sometimes they name them separately. Otherwise, there are a lot of factors that go into WiFi speeds such as the quality of antennas, and how many antennas there are. Typically laptops and desktops will have a 2x2 or 3x3 configuration. The iPhone 6s has 2x2 antennas, but obviously, there are a lot more tradeoffs that go into the antenna design in an iPhone vs something as large as an iMac.
If you are on a 5Ghz WiFi network (802.11ac) and get good reception, you should be able to speed test at near 300Mbps