It takes me somewhat less time to memorize a 7x7x7 than it takes me to memorize 10 3x3x3s. The reason isn't locations - they're about the same number of pieces/locations. The reason is that there are too many disjointed things to memorize for individual cubes for me. For each 3x3x3, I have to memorize edges, corners, flipped edges, and twisted corners. That makes 40 discrete sets of information for ten cubes. For a 7x7x7, I just need to memorize 2 sets of obliques, 2 sets of wings, 2 sets of X centers, 2 sets of + centers, a set of central edges, perhaps flipped central edges, a set of corners, and perhaps twisted corners. A total of 12 sets of information. It's larger amounts of information for each set, but the fact that there are fewer sets seems to help me. Somehow, reviewing that information is easier for me on a 7x7x7 than it is reviewing for 3x3x3s.
Lately, I've been doing lots of 3x3x3 multis with 9 cubes. I'm averaging around 25 minutes memorizing 9 cubes. On a good 7x7x7, I should be closer to 20 minutes memorizing. I'd say I get through the task of memorizing them in about the same time as a 7x7x7, but I take longer going over the memorization on the 3x3x3s than I do for the 7x7x7. So I actually get going on the 7x7x7 quicker.
For execution, the 7x7x7 is definitely slower than the 3x3x3s, for me. You can go through the 3x3x3s really fast; the 7x7x7 takes longer. Switching to BH wings has helped a lot on execution for 7x7x7, though - it's really helpful to minimize the move count on 7x7x7, where that's not quite as important on 3x3x3.