For many years we did business with a man who was Winston Wiser's groom and Merry Go Boy's exercise rider. He cooled out Go Boy after his multiple contests with Midnight Sun. He did not perform a "big lick" or anything like it. His animation however was dramatic. Ditto for Midnight Sun. If you watch these horses move and then watch a modern Big Lick horse the difference should hit you between the eyes like a 2 x 4.
The last son of Merry Go Boy stood at stud for many years in Oak Ridge. We bred a nice mare to him in the early '90s and got a lovely filly (the color of a newly minted copper penny). She had outstanding conformation and movement and temperament. But since she was not a "big lick" horse her economic value was minimal. She ultimately ended up in AZ with a decent owner.
In the Big Lick horse you swap "animation" for "action." The goal is to lift the front end as high as possible. What the rest of the animal is doing just doesn't count. It's a perversion of the high-step of the ASB or Park Horse. Indeed most of the Big Lick practices were lifted wholesale from the ASB world back in the early '50s.
As to speed, the flat and running walk are distance (or perhaps time) gaits vice speed gaits. A good flat walk should give you 4-6 mph and a running walk 8-10 mph. The form is exactly the same; what differs is a modestly increased reach (lengthening stride) and an modest increase in cadence (strides per minute). These are not "speed rackers." If the rider pushes the horse past these limits there is a dramatically high probability of losing the form of the gait. Once that happens you open the door to injury to the horse in the same way any athlete who sacrifices their form for drama opens them self to a significantly heightened risk of injury.
Go Boy was not a big horse. Midnight Sun was. Midnight Sun was also 1/4 or 1/8 German Coach Horse. Remember that the TWH registry was only formed in the mid-30s and in those days there were a lot of horses in the Registry that were there by virtue of performance testing.
If Midnight Sun had size Go Boy had stamina. After his second Celebration win (with it's legendary 30+ min. Work out) our friend was leading him out of the arena when a kid with a bunch of balloons suddenly came around a corner. Go Boy spooked very badly and my friend had some time calming him down. Midnight Sun, at the same time, was being kept alive by his crew as he was totally exhausted and threatening to die of the "thumps."
Personally I always preferred the Merry Boy/Merry Go Boy lines as they were a bit smaller, a bit tougher, a bit easier to train, and a bit smoother than the Midnight Sun lines. This was not what the market liked, however.
I never liked the highly exaggerated head nod. It looks cool in the show ring, I guess, but on the trail or doing any real work it's a pain in the butt.
Sadly, the Walker has been grossly damaged by the Big Lick preference throughout the industry. It's an open question as to whether that damage can be repaired. I suspect it can, but it will take several decades of good breeding to restore the horse of the 1930-40 era. That means big money and big commitment. I don't see this combination today.
This does not mean that there are not good Walkers out there; there are. We had many. But they are very difficult to find.
What did the Walker in was a triumph of a style over sound equine biomechanics. It's not the only breed/discipline where this happens, but it is the "poster child."