That must be some divine quickness I'm talking today. Breaking something at the speed of the god. Actually, neither of those word reference any sort of quickness or velocity.
Fasting is the act of refraining from eating. We usually don't eat during the night and break the fast in the morning. Quite logical when you think about it.
There are a lot of such words, at least for us non native speakers that are just that: words among others. The parts go unnoticed and you just think of it as one word. Especially when you are taught English doesn't really do compound words. At least not in a scale Finnish does for example.
Some of those aren't that obvious. They can be ambiguous like the fast in breakfast. They might be remnants from the older meaning like the speed in godspeed meaning luck or success instead of quickenss like its its commonly understood today.
There are also terms derived from other languages. Take mortgage for example. From French meaning the pledge of death. Sounds fatal, like you are trapped with the loan until you die. In this case the death luckily references the end of the loan making you free from the pledge.
Then there are those words that are so obvious. But you have always taken them as is. Like peanuts, which by the way aren't nuts at all. Buy what are they then? Doesn't leave that many options, doesn't it?