Yes, but within the acceptable range of fat bike tire pressures, say, 4 to 15 psi, dropping from the high range, which might only be acceptable for riding on smooth/paved surfaces, to the lower range that's best for very soft surfaces or snow, there is a remarkable decrease in speed for a given power output by the rider. Yes, 30psi will be slower and unacceptable for multiple reasons, but dropping from 12psi to 5 psi will absolutely make you slower on hard surfaces, although it also makes it possible to ride on terrain you can't at 12psi.