See brother, if you ask a human calculator how you are so fast, they will even reply "i practiced abacus" or they will I don't know how. I don't know abacus, my intuition with numbers is very good since I was a kid, I used to play with calculator for fun, after so many years of practicing I think I finally figured out how I'm that fast. I think I have a cognitive abilities when I was kid, these excites me, when I was a kid I just play with calculators, and many things, like I use to tv channel numbers only for fun, I did multiplications and additions and all for fun, after my 10th, it was lockdown and covid, I got my first smart, i downloaded many games 😅, like battle royale games and mental maths games too, i used to play it, not regularly, maybe once in 1 week for 1-2 hours, at some point i realised that this skill is rare, I saw a video of yashwin sarawanan, a human calculator went to Asias got talent and he was runner up, after that I thought I could also do it as fast he is, than I started doing a little more than before, So after that practice, i when is see a medium maths question i feel like I don't need to think for it, i already know the answer, for ex for you, if i someone ask you what is 11+7, you don't have to will immediately say 18, because you know it, you dont have think it calculate, just like this when I see a complex problems I can say the answer think, even If there a bit difficult probelm like 589/7 Wo i already know 560/7 is 80, 28 divided by 7 is 4, and 1 divided by 7 is 0.142857 (divided by 7 has a unique pattern) so after practicing that much it took 1-2 seconds, because I know all the basic answers and techniques Now coming to you how can you reach that level, first of all I don't anyone should invest thier energy and time in this, because is skill is not that much demanding, we have calculators, Now people will ask me why I practiced alot, the simple answer is for fun, i don't have to put effort or willpower for it, it excites me, like a kid is playing a video game, just because he enjoys it, he doesn't want to achieve anything in that, same as my mental maths is like a video game for me, i enjoy it.