I've got to say I'm puzzled by the concept.
A lot of what people describe as fast fashion is just high street versions of expensive brand designs. People talk about lack of longevity in the styles but in IMO that is more due to the style bought rather than the branding.
People complain about the materials however both use manmade materials both are a mixture of more sustainably sourced and less. Green washing is present in both ends of the fashion market.
People talk about construction. But if you examine how something is made and deduced all seams seem sturdy enough and it fits you well is it really fast fashion just because it is bought from the night street?
To me fast fashion is more about how you wear it. If you wear stuff for years it is not fast fashion. Full stop. Regardless of where it came from. I've got high street and supermarket clothes I have even bought second hand I have had for years and they still look good and I will continue to wear them.
I think fast fashion needs to stop being used as a justification for a) sneering at people who spend less of individual items of clothing and b) a justification for spending ridiculous amounts of money on clothing.
Well that's my opinion. What's your's?