In my opinion, production shooters are slower than L10 shooters because:
1) Reloads. The magazine pouch position combined with the lack of magwells tends to slow us down. This is not to say that a production shooter can't nail a reload, it is just a bit harder to do it consistently. One of my personal reloading problems is that I do not have an extended mag button on my production gun and every once in awhile, I have a hard time hitting the button.
2) Minor power factor. When each C costs you 2 points, you really have no choice but to slow down.
3) Grip. On my limited gun, I was able to modify the grip, grind the frame, replace the beavertail, and replace the trigger pad so that gun fit perfect in my hand. This helps in the draw, target acquisition, and most importantly in controlling recoil and driving the gun to the next shot. In production, you can't modify the frame at all and you can only change the grips to those offered by the factory.
4) Weight of gun. The weight of the gun can only be changed by 2 oz in a production gun. My limited gun is a long/wide frame plus a tungsten guide rod and all that stationary weight helps me control the gun.
Production shooters are not always slower than L10. Sevigny's Glock trigger is 100% stock, but that doesn't slow him down at all. Remember, Dave Sevigny did win one stage outright at the FGN (beating all the L10 guys as well as the production guys)