This question often accompanies a switch from one type of water softening salt to another, e.g. going from pellets to solar. What you are seeing perhaps is related to a difference in bulk density between the two products. The closer the crystals pack together, the less volume they occupy. This would give rise to the perception that the salt is dissolving too fast. In addition, if the initial water level in your unit was set to use pellets, it may be too high to use solar salt (i.e. too much water in the tank dissolves more salt) and may need to be lowered. Follow your softener manufacturer’s instructions.
In reality, salt can only dissolve to the extent that it produces a saturated brine (26.4% by weight). It doesn’t matter what salt is used. When the brine is saturated with respect to salt, no more can dissolve. Therefore, even though appearances may suggest otherwise, salt usage is the same regardless of the salt product type or form that is used.