The only metric should be pop time. Traditionally, the pitch location dictates the type of throw down. Low pitches, throw from knees. If the pitch brings the glove up, then throw from feet. A knee throw from a low pitch should be faster than throwing from the feet on the same pitch.
I'd be really surprised if the velocity is much different between the throws, release time, as indicated by Eric F, is the major difference.
I'm not sure what is meant by the bold. I have no issues with her pop time. I can see how it may be misleading because it is posted in the Catching forum but catcher is the only position that routinely throws from their knees. DD has a very strong arm but she loses a lot of velo throwing from her knees, at least IMO. I am just trying to figure out if keeping the same velo throwing from the feet compared to the knees is asking for to much or if it is obtainable.