I still think when it comes to the history of fast fashion, there’s always going to be an imperative to follow the latest trends even when hand-sewn (as it was for centuries). If it had been the case since before, it shouldn’t be surprising that the need to catch up with the latest trends in fashion more or less necessitated greater efficiency in production.
There could’ve been tailors and seamstresses who tried to speed up production by using multiple needles on the same garment (I know this from experience). The coming of sewing machines would’ve helped it a lot and still does today. This was good news to emerging fashion companies as they needed a lot of clothes to be done soon.
Keep in mind some of the problems likely existed earlier. Considering the nature of slavery, there could’ve been people who made others do the dirty work of growing and raising fibre plants. Sometimes even weaving and sewing them for others. Fast fashion practically exacerbated many of the existing problems.
It’s like if you wanted to grow cotton, you need a lot of land to do so and take care of the plants as much as you can (cotton needs water but some like linen needn’t much). Employing many more people to do it for you needs much more land and resources being spent. This goes quadrupled if you add in dyes and castoffs.
Some of fashion’s problems already existed before. The thing’s that fast fashion more or less exacerbated many pre-existing problems, be it wastewater from dyes, clothing castoffs or the resources needed to nourish cash crops (especially clothing fibres) that when you multiply this, it’s going to strain things anyways.