As some have said, Andrew, this small "power drop" (if it is indeed a power drop) is nothing to worry about. If the rifle is shooting nicely and cocks smoothly without any undue noises, maybe leave alone?

Were you using the same pellets on those chrono runs?

Those "loading lever slots" relate to the cocking slot on the underside of the cylinder. You're quite correct to not administer any lube through the transfer port. However, if you wanted to, in my view there would be no harm in applying a little moly grease to the cocking slot, any visible spring coils, the rear of the piston and a little through the back of the slot, behind the piston and onto the cylinder wall.