I'll tell you what I thinks makes for slow ignition in a flinter...

A touch hole that's too high...When I build a flinter I set it up so that the
touch hole is behind the heel of the frizzen when closed...You put that thing
down low where it can be covered with powder and the powder has to burn down
to ignite the main charge...

Where is this touch hole??? I've even made touch hole liners, counter bored them
and drilled them off center so it would be in the right place...I'd also look at going
to a White Lightning liner...

I don't care much for those cut flints either, I prefer the black English flints...