yes 100 mach.. and yet they get hit with pistol bullets and fight with swords.
So if you look over the incidental 'feats' that you need to take with a grain of salt because it's an unrealistic world, and just use your common sense you would come to the conclusion that they're just a lot faster than normal humans but nowhere near speed of sound or anything like that.
Do you really think Oda is calculation the speed of explosions or do you think he just wants Luffy to dodge it because he is 'fast'.
Fighting with swords proves really nothing, because the person behind it is fast as well. And pistols bullets aren't any problem for capable fighters in op verse unless there are special circumstances. We've seen this many times, Luffy have dodged bullets from close range numerous times. They've been doing this since the very beginning.
They never really show this kind of speed feats when going over long distances, probably for reasons like stamina but its still there and they can do use it while fighting and can cover short distances in with that kind of speed.
Most likely Oda did, because he too has some common sense considering its easy to understand that dodging an explosion like that after it just expanded requires the person to be very fast.. And its not the only example. And most of the rules of our real life also applies to his world as well. While people having supernatural abilities is indeed different, we can however calculate the effect of their powers, what it grants to them while comparing them to the things we already know. Ignoring them to me is not common sense just because one want to label the world unrealistic as a whole, why do you even bring up terms like speed of sound then? Are you sure that the speed of sound in opverse is the same as our world and that they are slower than that?
The feats are there and and they've been doing things like this for a long time now while keeping inconsistencies as low as possible. So I don't see why something like this present in the manga should be ignored.
If a character easily dodged something that obviously shows that he is faster. But how fast if asked can also be told from what he did.