This is a question  have been asked on occasion, so here is what I know:

The average dog has about 220 Mio olfactory receptors compared to the average human of just 3 Mio.

As such, by virtue of their nose, dogs live in a totally different world than we humans do. And, the scent levels that they can detect are minuscule.

This said, dogs can pretty much cruise through a room fairly quickly.  And that again will depend pretty much of the level of clutter as well as accessibility to all areas to be inspected for an average-sized room.

If you have a good handler, he/she will assure that the dog goes through the rooms in such a fashion that all areas are sniffed out.
Don’t equate time with quality/accuracy as your only metric of proficiency.