One thing that continues to mystify me about frame design, is how different bicycles react to weight distribution. Over the years, I've really found this aspect of a bicycle's handling to be a wildcard. My Rivendell Sam Hillborne is built like a typical mid-trail touring bike, which, it is said, makes it optimal for carrying heavy loads in the rear but only moderate loads in the front. However, I prefer to ride this bike with an enormous handlebar bag, and it does very well with it. It also handles well with a saddlebag and panniers, but interestingly the handlebar bag - with the same amount of weight in it - does not appear to affect speed at all, whereas the saddlebag does a bit. If I am going on a fast ride, I remove the saddlebag but do not bother removing the handlebar bag. All of this is just fine with me, because I find it far more convenient to keep my stuff in the front for easy access. But it goes against my understanding of the way a bicycle like this is supposed to react to weight.
Riding bikes like this has made me understand why some cyclists chose to ride with backpacks instead of baskets or panniers - some bicycles simply do not handle well with weight on either end, but do fine when the weight is part of the rider. This is one reason the argument "If you want to lighten your bike, lose some weight" does not make sense to me. You cannot simply take the combined weight of a bicycle, its accessories and its rider, and assume the handling and speed will be the same as long as the total remains the same.
My first city bicycle - a Pashley Princess - came with a huge front basket, but it did not handle well for me when I put things in the basket. Eventually I removed it and attached a set of rear folding panniers instead. With that configuration the bike handled much better, and faster. When I carried weight in the rear, I could detect no difference in speed, even with a full load of groceries.
On the other hand, the Bella Ciao city bike I now ride - while faster than the Pashley overall - is more sensitive to weight in the back. The handling doesn't change, but I can feel a difference in speed depending on how heavy the load I am carrying is.
With pretty much every bicycle I've ridden over an extended period of time so far, I've noticed some sort of relationship between weight distribution and handling, and it is not always a logical one - or at least not obviously so. I am sure there are lots of factors contributing to these effects, and these factors are just too nuanced to be obviously discernible. It's interesting to figure this stuff out in the process of getting to know a bike.