How to increase the growth rate?( Autoflowering)

You aren't far off from mine at 3 weeks. Most growth is still in the roots. I use the same power LED light different brand, height 18-24 inches at 75% power throughout veg.. I use Fox Farm soil so I still wouldn't have feed yet. I start all my grows in 5 gallon fabric bags and by this stage I would be doing a complete water of all the soil, the roots need it but I only have to water every 7-10 days. That changes quickly, by week 4 it's 5-6 days and by flower it's 3-4 days. Good luck

Hey Growmie, At 20–25 days, growth should be faster. Likely causes: overfeeding (All-Mix is already rich), too little light, or poor watering habits. Try ½ strength Fish Mix or plain water, keep light ~30–40cm from canopy, ensure temps 24–28°C, RH 60–70%. Hold off on LST until growth improves. Make sure they're in final pots to avoid root stress.

At this stage in life, they are growing exponentially (usually in ways you cannot see). You can increase the energy given to the plant in the form of light and optimize your environment. Most variables in the grow room come down to the air and medium properties. After that, altering the photoperiod you are giving it. I have the most vigorous growth when I can gently hand over a hefty DLI to the plants while maximizing their recovery time. In photos this usually means a strong light intensity during an abbreviated day, with a prolonged dark cycle. This is going to be somewhat counterintuitive to the bro-science folks, so play around with it and see how the plants behave. Fact of the matter is that there is no other plant on this planet with such substantial genetic diversity, no plant on this planet with more genetic selection happening, and no plant cultivated so widely with so little bonafide scientific research.