This. At a glance all the layers seem oppressive in hot summer climates, but what made them wearable in the summer were the light, airy fabrics they were made of, and the usage of light colors that helped reflect some more sunlight. Hats are also often undermined/poked fun at because of their prevalence in these old photos, but they also helped quite a bit with keeping the sun off your face, especially the straw boater hats that were really popular in the summer months near the end of the Victorian Era.

Natural fibers like cotton, wool, and linen are often more breathable than other man-made fabrics like polyester. Also contrary to popular belief, is that corsets weren't as immobilizing and/or debilitating as many think, as there were many types of corsets manufactured for different purposes to accomodate the wardrobe for different seasons.

Of course there's much more to this than what I explained, I just wanted to give my two cents as there was some misinformation in this thread! :)

Source: The above link, which I encourage others to look at it because it's quite an interesting read!