system
Given the following post, respond as if you're commenting on Reddit:
human
[Dragon Ball/Z/Super] Why can someone like Krillin get so much stronger then someone like Mercenary Tao?

They're both humans and they both had A1 training (Tao at least trained under Shen, possibly under Mutaito), but Tao is over 300 years old. Why was Krillin able to overtake him so quickly?

Within 3 years of training with Tien, Chiaotzu, and Yamcha off in the wilderness he already overtook Tao. Past that, with the one year of training between Raditz and the Saiyans arriving, Krillin massively jumped again making Tao almost laughable.

What is it about guys like Krillin, Yamcha, Tien, and Chiaotzu that makes it possible for them to grow so much more powerful than someone like Tao, Shen, or Korin?
gpt
i think it got something to do with them being around powerful beings and the ability to sense ki. 

what they are doing isnt training their muscles, they are training their ki. and considering that stronger people increase in power much faster (by sheer numbers atleast) than weaker people, they are probably doing something with their ki that the weaker persons arent. 

so if you can sense ki, and you train with someone strong, you can sense how they move their ki around and what they do with that, and you can immitate that. krillin trains with goku, and he can learn from goku how to do ki stuff better. im assuing this is also why traning with superpowerful dudes are relevant, since by now they know every way to there is to lift a weight or throw a punch. i also assume this it got easier and easier to become super sayian and unlock those other transformations other seeing others doing it, culimating in the u6 sayians who learned it very quickly from goku and vegeta. i think roshi even mentions it to krillin in ressurection F in dbs, that just by being around so many superstrong beings, krillin grew acustomed to their power, and in turn got stronger himself (and especially faster, which that scene is about) 

case in point, goku trains  with king kai and gets a lot stronger, krillin and the z fighters learn from goku, Tagoma got a lot stronger from training with freiza, future trunks in dbs by fighting with goku and vegeta against black, and goku and vegeta from whis and so on. train with someone stronger, and you can learn how to become as strong as them, as long as you can sense ki to see how they did it.

so what tao and shen and the others lack was a guy like goku, who could show them how powerful you can become and help pull them up to that level.