I always get the largest healthiest plants by starting with a larger pot, not transplanting later beyond the starter soil indoors. If my climate/grow season length allowed for it I would even direct sew seed into the large pot soil to avoid any transplanting.
I suspect it is merely an optical illusion if you put a plant in a large pot and think it is growing slow, that it's just that you have a small young plant and a lot of pot around it. If it is growing a lot of roots due to the larger amount of soil, that directly benefits growing a lot of leaves and stem. It's not as though if it has a chance to grow a lot of roots that it would grow fewer leaves and stems, or eventually pods, rather the opposite.
The reason a larger pot is so important even for a small plant is that this allows minimal watering to maintain an adequate level of soil moisture. This keeps the soil from compacting much while the roots get started so that the roots suspend the soil and it always stays fluffy for better aeration and drainage. The smaller the pot and more often you have to water it, the less you will have this ideal. Transplanting later does not make up for this, it will still help but then the compact root mass is depleting nutrients from a smaller area still until it grows into the additional soil.
5 gallon pots are only good for short growing seasons. That is the undersized pot size, not the larger pot size it is best to aim for. 1L pot the first season only serves to dwarf the plant. Their picture shows about 10 pods on a plant that must be over 2 months old to already have them ripened. If you want 10 pods instead of hundreds then use a 1L pot.
