An increased swing speed increases your break speed if the cue stick weight stays the same. Increasing the mass (weight) of the cue also increases the speed of the break if the swing speeds stayed the same. I think most everyone realizes these facts. Both mass (weight) and swing speed contribute to the break speed. More mass and more swing speed are both good things.
Also a fact that just about everybody realizes is that as the stick weight increases your maximum swing speed decreases, and vice versa. If you increase mass (cue weight) you will decrease your swing speed. If you decrease mass you will increase your swing speed.
But since both speed and mass are good, but increasing one decreases the other, is there a perfect sweet spot in there that is the perfect trade off between swing speed and cue mass that will give the highest break speed for someone? Yes, but this optimal weight is different for everyone. You have to experiment to find what it is for you.
What I have found is that people that swing the cue much slower than average typically do better with a heavier break cue (closer to 20-21 ounces sometimes). Yes the heavier cue slows down their swing even more, but on net they tend to pick up more break speed from the increased cue weight than they lose from the decreased cue speed. And people that can swing their arm much faster than the average person are the opposite. They tend to pick up more from the additional speed that they can swing the even lighter cue (closer to 16.5-17 ounces sometimes) than they lose from the cue having less mass.
If you can't swing a cue very fast, you will probably have a faster break with a break cue that is heavier than average. If you can swing a cue much faster than most, then on net you will probably have a faster break with a break cue that is lighter than average. And if you are like probably 75% of people who are somewhat more typical of how fast the human can swing their cueing arm, then the cue you will have the fastest break with is also kind of in the middle somewhere not too far from the 18-18.5 ounce range.
As far as control goes, that also varies for people but in general you start to lose control with really light or really heavy cues either one. There are probably few if any people who will have their best break with a cue that is below about 17.5 ounces or above about 22 ounces, and many people won't have good control anywhere even near these weights.