Notice the developers didn't slow down the reaction speed, they just knocked it out. And as you know, putting a rocket into a crack also requires a reaction and this was not removed.
But this is not the point, the wave scanner should be guaranteed to indicate the presence of the core and if it does not do it is wrong. When mining cores there are so many other factors - complicating mining - to still tolerate the scanner not working correctly.
Imagine you scanned a ship, you were shown that it was a pirate, opened fire and immediately got a penalty, the scanner was wrong

And you realize that the brightness of the asteroid after lighting depends on a bunch of factors, maybe the person is playing in a bad resolution, his monitor is dim, etc.
Some people have their graphics set to ultra and some people have their graphics set to low.
Even in SRV the scanner works more correctly !