"My understanding is that it takes longer to grow CBD-rich indica strains than THC-rich sativa strains"
Other way around actually. Indica strains grow faster and are easier to grow. Most street cannabis strains are hybrid, where they are trying to get the high THC of sativa but with the favorable growing characteristics of indica.
It's fair to say I'm no expert on cultivating cannabis!
I think the idea more generally (can't remember where I heard this, think it was one of the many documentaries focussing on dispensaries) was just that those growing 'street' weed aim for high THC and don't care all that much about CBD, while those growing it for legitimate medicinal purposes aim for high CBD with as little THC as possible.
I think I see where the confusion is. The Indica "brick weed" that's commonly sold is typically imported from countries where it grows naturally in the wild. The Sativa strains you're referring to tend to be grown in the setting of domestic indoor operations. The Sativa strains are more expensive and are in higher demand and are higher risk due to a) the time that it takes to grow them and b) the fact that it's being done in a high security and low corruption country. This provides motive to grow them as quickly as possible and make them hard hitters when smoked. You want to produce the most "bang for buck" in terms of the physical space you have available, so high concentrations of the most noticeable compound (THC) are favourable. There is also a higher correlation between hardcore drug use and those strong strains, whereas more casual users are likely to buy the Indica brick weed. All of those documentaries are probably referring to the hardcore users, not casual users. They are addressing a real concern but gloss over the fact that the reality is more complicated than it appears on the surface.
"those growing 'street' weed aim for high THC and don't care all that much about CBD"
People growing weed illegally for commercial purposes usually care more about weight and grow time than anything else. Of the actual breeders and geneticists who create the different strains, there are a variety of different factors that are optimized for. E.g. check out the webpage for apothecary genetics:
Most of the good breeders are in the Netherlands, but they are probably the top ones in the US.
"those growing it for legitimate medicinal purposes aim for high CBD with as little THC as possible."
That's definitely not true. The vast majority of medical marijuana is high THC. While many dispensaries are starting to carry high CBD weed and extracts, they'll usually only have one or at most two strains out of the 20+ they offer. In terms of actual medical use, different cannabinoids have different medical properties. So THC also has many medical uses, it's just not the only cannabinoid with medical uses.
I'm actually startled that this might become the new debate: what strains of marijuana are 'good' and 'bad' and which in the cannabidiol group are 'good' and 'bad'. People actually have serious conditions that are treated specifically by THC, which is capable of adjusting serotonin levels. This is significant for a number of conditions such as migraines and depression.
Please do not generalize like this because it's actually harmful to active patients.
Other way around actually. Indica strains grow faster and are easier to grow. Most street cannabis strains are hybrid, where they are trying to get the high THC of sativa but with the favorable growing characteristics of indica.