If fed the right foods and given clean water you will have better growing fry, healthier fry and more colorful fry.. as you are giving them all they need to thrive.
By not doing the proper water changes, nor feeding enough/not feeding the proper food you are just stunting them, causing them to grow slower.
Yes, naturally bettas can take 9 months to mature, but that is in the wild where food is scarce, stress is high, etc etc. Which can play a factor.
I have heard accelerated growth can have some bad side effects.. but honestly.. I personally would rather my fish have clean water and good food and let them grow however that type of treatment allows them to grow.