Take your 4f powder used for your pan and grind it up using a morter and pestal into a fine powder. If your picking the vent hole after you load it, it will hang every time. Doing this pushes powder away from the vent hole. With a big vent hole you can pick after the shot before loading and then use the fine powder and make sure you evenly distribute it in the pan and some gets in the vent hole. If your cleaning between each shot, this might get some water or cleaner in the touchhole or in the breeech area, which will slow down times too. Do not use too much powder in the pan. Too much powder will make your lock times longer, but hey, a full pan of powder sure looks great for those camera shots. Clean your pan between shots on humid days. Simply brush it and maybe wipe it with a dry cloth.

The other thing that will decrease lock times, not vent hole related, is to polish the inside(and outside) of your lock. I usually get some jewelers rouge and polish it on a buffing wheel or using a dremel tool with the buffing wheel, which can be difficult because some of those parts are small. At any rate, any part that comes in contact with another piece of metal gets polished to a mirror finish and then oiled with your favorite gun oil. This will slightly decrease your locktime and make your lock feel much smoother. You will have to take the lock out to periodically clean it, especially if you have been hunting in wet conditions. The only part that you really want to do on the outside of the lock is where the frizzen cam rubs against the lock. Some locks with roller bearings where the cam is you really don't need to polish this spot.

I'm sure you know this already. Just saying it in case you missed something.