This has been a solved problem for many years. My 20+ year old Velodyne 18 inch sub is -3 at 14hz and distortion is <1%. People who think big subs are slow should turn off their mains and listen to just the subs. There is nothing fast sounding <60hz.
I experimented with a bunch of smaller subs to see how it worked and I wouldn't recommend it. I had 2 10", 2 8", and 2 little PSB sub-series 100s. The surface area was equivalent to a 20" sub but it didn't come close to the big Velodyne. The excursion wasn't anywhere near comparable. I understand having multiple subs to help with room interaction but the bigger the better.
My experience with bass speed and impact is that it has more to do with mid and upper bass energy coming from woofers rather than subs. I've got a pair of ATC 110s and those have that in spades. They sound so powerful and dynamic, way more than any other speaker I've heard. But they're not flat to 20hz or anything even close. They each have 2 9" woofers with 275 watts to each driver in active configuration.
I consider an 8" woofer to be the minimum for satisfying bass. I've tried speakers with 6.5" woofers and subs. It works fine for some music but if you want to crank beethoven or AC/DC it never satisfies.