First of all, Invisalign is a wonderful system. However, it works in an entirely different manner than traditional braces does. It "pushes" teeth. Akin to squeezing a watermelon seed out from between your index finger and thumb. As such, tooth movement with Invisalign has to be deliberate and staged precisely. This means that not all teeth are moving at the same speed or velocity. Some teeth may not be moving at all. Typically, the most difficult tooth to move (usually the most crooked or rotated tooth) is going to be staged to move the slowest. Therefore, it may take longer with invisalign to see correction in certain problem areas.
You will usually see quicker initial results with braces because all teeth are moving in a relatively unprecise random manner. Even teeth that are straight, initially, sometimes are moved out of alignment with braces by the more crooked teeth only to be moved back to their or original position later on in treatment. In orthodontic circles, this inefficient mode of treatment is called "round tripping". This is not negligent orthodontics...it is just the way teeth respond to braces and wires.
Because of this inherent inefficiency in traditional braces, Invisalign treatment usually ends up taking less time to complete. I know this doesn't make sense but our average invisalign case takes approximately 12 months plus or minus to complete. Full braces cases average 18 to 24 months.
Please note that there are limitations inherent in any treatment option. It is very important for patients to ask questions so that their treament goals are met.