I'll throw out a couple of points for consideration.
Concerning that noticeable delay on firing, it may be the old powder. Pyrodex, like most (all?) of the synthetics is subject to deterioration from absorption of moisture from the air after the bottle is opened. Sometimes things are dry enough there is no deterioration, but you might try it with fresh powder, or real BP if possible, and see if there's a difference.
Concerning the roughness in the bore near the breech, collect some of your fired patches and look for signs of gas leakage in the grooves. If you have some burning, a tighter ball and patch combination might take care of it, or you might need to use something between the powder and the patched ball to seal it - a wad, an extra patch run down before you load the ball (I often use ones I picked at previous shooting sessions in my .50), or a granular filler such as grits or corn meal. I have a smoothbore with rough bores, so I usually use some sort of paper shot protector. I could see occasional burned streak on the recovered shot protectors from gas leakage if I didn't have a thick enough stack of cards over the powder (3-4 over-shot or cereal box cards, or 5-6 milk-carton ones). I found this before I tried ball in it, so I've always just used the cards and cushion under a thumb-seatable patched ball.
The pitting is going to accumulate fouling more than the smooth sections of bore, and is going to mean some more effort in cleaning than a perfectly smooth bore.
Joel