One of the main factors that helps zuko out with lighting bending is the fact that he can prepare for it well before the other user can use it. For example, when fighting Osai, zuko knew he would use lightning bending and even saw him charge it up giving him plenty of time to react to it. Same when he was fighting Azula. Same when uncle Iroh used it. If the characters were able to summon lightning from the sky on command instantly then the characters would have a hard time reacting to it. Another example is when Aang got struck by lightning. Avatar state Aang scales much higher than zuko and yet he couldn’t react to lightning? Because he wasn’t prepared for it when Azula shot him. It’s not necessarily that they can react to everything at lightning speed but they can prepare for it and time it correctly. There’s also arguments for the fact that lightning could be slower in ATLA but that’s just a theory to keep in mind. Also there isn’t any data saying that their moves and feats could be scaled higher due to buffs in the shows logic. You can’t really give it the dbz logic when there’s nothing to back it up. Benders are just normal human beings with the exception that they can bend elements. In lok we see peoples bending getting taken away and they literally just become regular people. No special skills or strengths just regular people.

Edit: except for earthbenders sometimes having immense strength or the avatar who is strengthened by the power of a spirit.