Another thing to consider when talking spinning speeds is the duration of your spinning - because it's a difference whether you are doing a sprint, or whether you are running a marathon.
For my spinning work, there's an amount of time I can comfortably spend spinning in one go, and there's an amount I can comfortably spin in a day. Once the individual sessions get longer, or there's more sessions in a day, it gets, well, no so nice. After all, it's prolonged sitting in one position, with limits on how much you can vary it, and it's the same repetitive motions again and again. I've had one reconstruction where things went south with the first try, so I had to spin the same amount of yarn once more, and at times there were grooves in my finger where the yarn ran over it. Not fun, I can tell you.
So if we're looking at production spinning, the sweet spot would be a spinning speed that allows the spinner to make as much yarn as possible in the time available, but still at a rate that feels comfortable and sustainable. You can spin fast for a bit, and you can certainly gain a bit of speed with practice, and it's also possible to tweak speeds some more - but in the end, there will be a limit to speed gains.
If you're looking at ethnographic sources and see spinning videos from other places, where hand-spinning is still done for yarn production purposes, the spinners don't appear rushed at all. To me it often seems like there's no pressure to get the task done as quickly as possible, or even pressure to get the task done - it's spinning, it happens, there's time for it while chatting with others, or walking somewhere, or herding animals or whatever other things are going on in the same time (or not).
Was it the same in medieval Europe? I'd suspect that more or less it was. Of course there's an expectation on how much one spins in a given amount of time, but there was probably not the hardcore mindset of "time is money, work quicker" that we tend to have today. We'll never find out, and there will also have been individual differences (if you spin for money as a service provider, you'll probably look at it differently than when you're spinning for yourself while you have the necessary fabrics for now, and are just making sure you're not going to run out of fabric in a year or so).
It always boils down to the same thing, though: There's a speed that you can comfortably sustain over periods of time, and that will make sure your yarn stays consistent in quality. This is the speed that one should look at for production speed, and for speed calculation. That speed will also depend on whether you're doing something else at the same time (even if it's only chatting!) or if you are concentrating fully on your spinning, as well as on all the other factors discussed previously. Spinning as quickly as you can is definitely not the speed you'll be able to hold up over several hours, just like you won't be able to sustain a running speed for a sprint over the distance of a marathon.