Slow can be good for some species, but you may also be inviting stain, mold and will not get rid of any insects. In northern latitudes, using solar to dry is a small step up from air drying. Conventional kiln drying is an effective method to reduce moisture in lumber uniformly and in a timely manner. The best attributes of conventional kiln drying is that it does not require expensive equipment, and can use a host of energy sources to save money. This method also allows you to inact any kiln schedule from slow to fast drying, allows temperatures to kill insects, and allows conditioning of the final product for relieving stress.
Moisture leaves wood from the outside first and then the inside.
This transitional zone between the core and surface is where stresses can occur. If too great, they can cause the lumber to split and crack on the surface and interior of the lumber. The speed at which the lumber dries is directly related to the stress imposed on the lumber. The best weapons you can have in your arsenal in the war against lumber defects start from when the tree is felled. End coating logs and sawn lumber before it reaches the kiln is a good hedge against end checks.