I have no idea on how to set up the passenger speed and yes it is by default very slow...
The passengers speed is not slow, and cannot be increased. As I've said in my previous message, the overall speed of the entire boarding process is directly affected by the Passenger Density slider.
And of course, on a gate, if there are two jetways, it will take half time, just like in real life. And on an apron, with a plane with 3 passenger stairs, it will take 1/3rd of of the time required compared so a single stair.