The original sentence has an error in parallel structure, as it uses "more efficient" but does not use parallel structure for the other two items in the list. Also, the comparison structure is not correct as it doesn't use "than the previous model" to compare to the previous product.
Option A corrects the comparison structure, but the parallelism is still incorrect as it uses "but it also uses less energy" instead of the parallel "and has a faster processing speed."
Option B also corrects the comparison structure but still does not use parallel structure for the last two items in the list.
Option C corrects both errors by using parallel structure for all three items and also correctly compares the new product to the previous model. It uses "is faster in processing speed" instead of "has a faster processing speed," which is more parallel to the structure of "uses less energy."
Option D corrects both errors and uses parallel structure, but it switches the order of "uses less energy" and "has a faster processing speed," which makes it less parallel than option C.
Option E corrects the comparison structure but is not parallel, as it uses "and is faster in processing speed" instead of "and has a faster processing speed."
Therefore, the correct answer is C.