Wi-Fi slow on iPhone, fast on computer or tablet
The following observation indicates to a problem specifically for iPhones in contrast to e.g. iPad and maybe related to your problem.

I had problems to convince my iPhone 6s to use the 5GHz band. iPad and iMac did it at once after I switched off the 2.4 GHz in the router. The iPhone would not connect, even after resetting the network settings. The trick I used (not my own idea) was to use two different SSIDs for 5GHz and 2.4Ghz. Even then it took two or three attempts, but now the iPhone connects to the 5GHz.