I recently drove from Canada to the U.S. with someone from a Visa
Waiver Program country. The border agent in the booth said he had to
go into the office to fill out a form.

The form turned out to be a standard I-94W. It took half an hour
until he was done. There was nobody else waiting in front of him, and
nothing complicated about the situation.

Does it always take this long? Is there anything travellers can do to
speed up the process?

If it's a crossing that handles cars and pedestrians, and there's a
sizeable line of cars, how would they take it if a passenger subject
to this requirement got out of the car at the back of the line and
walked into the building? Then we wouldn't have to wait on the car
line and *then* wait for the I-94.

Jimmy