Sorry, Luke and Han are looking at explosions not aiming, and if you actually watch it they are firing in the right direction before the TIEs are even in visual view of the windows.
You're wrong on the targeting also, Luke and Vader pull a trigger and only when the targeting computer displays a lock. They aren't aiming, they are simply making the decision to fire.
Physics doesn't take a complete holiday in my games. The human eye can't perceive something moving at thousands of mph at the distances in these clips and it certainly can't make the brain react in time. If people can react to targets moving that fast in your game then there is no reason they can't see a bullet in flight, because bullets move a fraction of the speed of these ships.
These are physics in our universe, it's a sci-fi fantasy world and all things aren't held equal. If we want to go by the physics of our world, the majority of things in StarWars wouldn't make sense, at all.
Targeting computers are mentioned here:
It's nothing more spectacular than anything than something in fighter planes from the 70's and 80's. If you notice it says the target is 'vulnerable' when the sights are lined up.. There is no 'locking', Proton Torpedos 'lock', the blasters still have to be lined up and aimed by the pilot, or gunner like a standard 'sight' - the difference here is the siting is digital. The concept is as simple as the gunners tracking sensors movement along the ship and having knowledge of the firing range. There are plenty of misses by Luke, Han and Vader, because they have to use their own hand-eye coordination, piloting skills and lining up sights to hit a target with Blasters and Lasers. This is why they aren't looking down, at the computer - their eyes shift to space when a TIE gets in range, and back to the computer when they make a kill shot.
Here's an excerpt from the IN-344-B "Sightline" Holographic Imaging System used by the X-Wings in A New Hope:
The device simplified what could be seen by the pilot, making computer-assisted targeting a much easier task. However, it also possessed a chance of causing a near-miss, as evidenced by Garven Dreis, the "Red Leader" of Red Squadron during the Battle of Yavin, firing off two proton torpedoes at the thermal exhaust port during the Trench Run upon getting a lock, but instead hit some yards away from the exhaust port
It says specific 'making computer-assisted targeting a much easier task' implying that the systems used on Targeting Computers aided in making firing decisions, but reliance on skills and sighting down targets without the systems were the primary means of operating lasers. It also indicates that systems like this were used more often with weapons that could 'lock-on' such as proton torpedos.
Also an excerpt from the ANq 3.6 Tracking Computer:
The Fabritech ANq 3.6 tracking computer was a targeting computer commonly installed in the Rebel Alliance T-65 X-wing starfighter and the B-wing starfighter. Information was relayed to it via a multiplexer array. It could track up to 1,000 moving sublight objects, acquire 20 possible targets, and could be programmed for extra sensitivity to 120 specific sensor signatures. As such, its tactical imagery was commmonly used in X-wings to target, aim, and launch the X-wing's payload of MG7-A proton torpedoes.
Here's St2x Targetting System used by Xizor's ships, which is probably the closest to a sophisticated targetting system for laser weapons, but it's not explicitly mentioned. I'm going to assume they are effected because the article mentions 'all' weapons:
The St2x targeting computer was a highly advanced device used on Prince Xizor's Virago. Rather than developing new and potentially illegal weaponry for use on the Virago, the MandalMotors design team focused their energies on making existing weapon systems more effective. Their final design combined the very advanced St2x with an experimental laser sighting system. Once installed, the package was capable of providing targeting information to the pilot even while undergoing complex maneuvers. The system was so advanced that virtually any weapon systems installed on the Virago would perform at levels far higher than their manufacturer's specifications.
This is a one-off case of super advanced targeting computers on one ship, in the EU.
In any case, I'm bringing this up because the reliance on targeting computers for gunners to sight out laser shots isn't something present in StarWars and also trying to place our physics into StatWars isn't exactly a fair way to explain why or what things do in StarWars. That's like trying to explain why Dragons or Magic exist in a non-sci-fi Fantasy game. George Lucas could simply say, humans in the StarWars Universe have been using this technology for so long, they've adapted to sighting things that move at 1000's of miles and hour, and we'd go with it.