@Silwith I disagree. The big difference here between 10 years of training the sword in this dimension and the 45 year old swordsman that trained in real life is the amount of time that both actually used to increase their sword skills. Both need 8 hours of sleep, and let's say 30 minutes to make and eat breakfast on average, and 30 minutes to shit, take a shower and all other bathroom business on average.
Now the former spends the rest of his time on training non-stop, of course it's only by himself and he has no opponents so that's a huge downside, but on the other hand he perfects his posture, swings, every single motion to 100% efficiently cut with the sword. And you must remember, this is uninterrupted, full-focused training, no need to think or worry about anything in the real world so they can put all their heart in it.
As for the latter on the other hand, they have a life they need to live before training. He has 15 hours, I'll be generous and say he trains for 6 hours a day on average, perhaps 3 hours in the morning and 3 more hours in the evening. 6 hours of continuous training is FAR better to having them seperated into two different periods by the way. Every other hours he has a life to live, maybe studying or doing some sort of job. Let's say for two to four years he goes to war, that of course brings him a lot of experience, but how much of that time spent in the war is really spent on cutting people? Even then, battling other people has diminishing returns, it doesn't drastically improve your posture or the handling of the sword. What it does do is make you vastly better at utilizing the techniques you learned against other people and it makes you better at reacting and predicting/countering what other people will do.
What I'm trying to say here is, the latter is a better killer, but not a better swordsman than the former. The former has trained all those ten years continuously to make his thrusts, slashes and all that sword movement as effecient and fast as possible. All that efficiency is a technique all on to itself, not to mention 10 years is enough to make plenty of new normal techniques.
People underestimate how much life gets in the way when trying to improve your craft and don't understand how significant uninterrupted, worriless, full-time training actually is. So the fact that the author wants to subjugate the MC to 100 million years of this is not only retarded, it's also pointless. 10 years would have been enough in reality to get the same ability he displays here (taking into account all this magic shit as well), just not the 'cutting the world' bullshit because we have no idea what that actually means in practice.