How much faster does a mountain bike go with slick tires vs. knobby tires
Assuming everything else is the same, the wattage lost to best-rated road tyre test is 7.0W
Lowest-loss MTB tyre is 19.1W
Gravel tyre is 20.8W
Touring bike is 17.1W
And the Fatbike is 36.9 W
None of these tyres is great at anything but having a low rolling resistance - this is gained by sacrificing tread thickness and puncture protection.
There are plenty more reviews and tests at the same site.  Whether its real-world doesn't matter too much - each one is tested the same way for consistency.

Then the second part is &quot;how fast do these wattages allow?&quot;  Using http://www.bikecalculator.com/ and presuming a 75 kg rider, 10 kg bike riding at 150 Watts, will roll at 28.26 km/h (approx)
If this speed was done on the fatbike tyres, then moving to the touring bike would save 39.6 Watts, essentially increasing the rider's input power by that much, and returning a speed of 30.96 km/h
Likewise, going to the fanciest road tyre would result in 32.18 km/h
Since a bicycle has two wheels, losses are doubled.  Some of these tyres will cost hundreds of dollars too, and may be tubular.
A common GP5000 from Continental is measured at 10W, resulting in 31.82 km/h, only 350 metres/hour slower than the costly Vittoria.