Using a dull blade or the incorrect type of blade can cause the band saw to cut through work pieces slowly. The blade will also move slowly through the work piece if the blade is installed incorrectly. A weak drive motor can also cause the band saw to cut slowly.
First, check the installation of the band saw blade. The blade teeth should be facing downward-reinstall the blade properly if the blade teeth face upward. If you have the blade installed correctly but you have to push hard on the work piece when cutting and blade twists during cutting, then the blade is likely dull. Replace the blade when it's dull.
Use the right type of blade for the work piece material that you're cutting. For example, use a 6 TPI (teeth per inch) saw blade when cutting pine wood and other soft material instead of a 14 TPI blade because a blade with fewer teeth will cut through soft material faster than a blade with more teeth.
A weak drive motor slows down when the blade cuts through a work piece so replace the drive motor if it won't maintain the right speed while cutting.