Some locks are fussy and I try to avoid them. That fussiness is seldom talked about but a deal breaker for me. If the flint has to be exactly this length and no more than this thickness, I am guessing our ancestors would have been unable to fire their guns about half the time.
Added in edit: I also think the less expensive and less specialized the gun, the more a large lock makes sense for reliability under non-laboratory conditions. In my thinking, a Manton lock could be as particular as it wants to be because the man owning it would have access to the very best of everything including flints and powder. For the militia volunteer or even the longhunter, reliability of fire under adverse conditions probably trumped speed of ignition. Big lock, big rock, big frizzen, big pan, strong springs usually equals a lot of sparks and good pan ignition.