Is it possible to display my Mac on my iPhone using a USB cable?
They are the only options as of now. They are slow because of your internet. 

Even USB have speed limitation. 

And using a USB cable 6 or 12 feet long would defeat the purpose (you are to close to the Mac for any benefit).

Your WiFi speed should be good enough to watch streaming Videos without delay showing > 200kb/s.

To check, click on the WiFi while holding the alt key, that will open more detailed information on your WiFi.

In there it should say

MCS index >10