How fast does lettuce grow in a high tunnel aquaponics system?

Last week I transplanted some lettuce seedlings from planter cells into the gravel grow bed. I didn't use all of the seedlings, so I kept the leftovers in their trays. As you look at the following picture, keep in mind that all of these plants were the same size less than a week ago. The ones in the trays have grown a bit since then, but nothing like the lucky ones in the gravel.



Here's a pic of the same section taken 4 days ago:


So how fast? Really fast.