Hi Rich,
I do believe that you are correct about the speed of ignition being important in offhand muzzleloading.
The term in my mind is "dwell time", or the time from when your finger breaks the trigger to when the ball leaves the muzzle. The longer the dwell time the more chance for the muzzle to aimed where you DON'T want it when the ball exits. Everything from ball velocity, to lock speed, to flint type and condition, to touch hole diameter, to priming powder, to cleanliness of pan and flint, to barrel length will all affect offhand shooting. It is all about how fast you can get that ball out of the barrel while the sights are on target. In my opinion a swamped barrel helps keep that muzzle hanging there while the ball heads down the bore.
If I was making a dedicated competition offhand gun (ignoring traditional looks), I would use a 30" barrel, medium heavy swamped 40 caliber and add a Chambers Late Ketland tuned by Brad Emig. The small 40 ball can be driven fairly fast with little recoil, and the short barrel will reduce the time to get to the muzzle. Also, the Chambers Late Ketland is a very fast lock. Throw in a good set trigger and I would be happy. Something like a Lancaster style stock works best for me. Just thinking out loud. If I was REALLY wanting to win matches, the ignition would be percussion (but I love flintlocks a lot more). Even the best flinters cannot hang with percussion in speed of ignition, unless something is wrong with the cap gun.
So yes indeed I believe lock speed is a key component to good offhand shooting. That is also why I use 4f Swiss for priming. Pletch's excellent testing clearly shows 4f is faster than 3f, which is clearly faster than 2f in the pan. Will 2f make a gun go off? Yes, but not as fast as 4f. NullB is fastest, but in my climate it plates the pan with hardened graphite, so I dropped back to the slightly slower 4f.
In modern suppository weapons the barrels are short and speed is so high that little of this comes into play. That is why the bench technique for modern weapons will trip up a newcomer to muzzleloading. They don't realize they need to control that long barrel until the ball exits.
Long winded answer to saying "Yes, lock speed is an important contributor to good offhand shooting."
God Bless, and Happy Easter to All, Marc