Handspinning speed is one of those factors that need to be considered when spinning different fibers and/or when making thin yarns as opposed to thick yarns. Thin yarns, because there are so few fibers in a strand of yarn, need to be spun at a fast speed, giving them a lot of twist, so the relatively few fibers hold together. Thin yarns also have less air in them because there are so few fibers that there's little room for air space between them, and this can give the yarn a polished quality, although that's not necessarily bad.
Fine yarns are typically "worsted" spun whereas bulkier yarns are "woolen" spun. Picture a wool sweater with thick-ish yarn, yarn that is somewhat fluffy with air between the fibers. Part of the warmth in the sweater is due to the air trapped in the yarn. Now picture a summer weight wool suit, with finely spun worsted yarn with no air between the fibers, and now you understand why summer-weight wool is so cool!
Anyway, back to the matter at hand, I've been working with cotton fiber lately, 100% cotton and a cotton/silk blend. I've spun mostly protein fibers in my career (animal fibers) and have stayed away from cellulose (plant) fibers largely. Cellulose fibers generally need to be spun much more slowly than protein fibers because it's so easy to overspin them. The quality of the fiber is different, it needs less twist to hold together, can become kinky and hard with too much twist.
Both these yarns are 100% cotton. The one on the left was spun at a faster speed than the one on the right, and you can see kinks in it as well as a shinier finish and an overall harder quality than the one on the right. The "hand" of each of these yarns is totally different. The one on the left is less likely to be used to make something than the softer yarn on the right.
Perhaps the point I really want to make with all this blathering is that after all these years of fast spinning, I am slowing down! I have been spinning very fine yarns, mostly silks, for years, and even with plying them for 2-ply yarns, they're so fine it's hard to use them for anything but the sheerest fabrics. So now I'm beginning to slow down, spin yarns somewhat less fine, and get back into the meditative quality of spinning, rather than just see how many yards I can get finished in some period of time.
In short, I suppose you could say I am beginning to enjoy the journey now, rather than merely relishing in the results of my labors.