When trying to mine harder blocks, for example ore, the breaking animation starts and the block will start to break, but after a fraction of a second the mining gets interrupted and starts all over (similar to when a bat gets in your way and interrupts your mining). This can repeat for 2 or 3 times before the block actually starts to mine appropriately.

It is most noticeable when switching focus from one block to another or rather when you move your cursor from the block you've just mined to another one you want to mine.

I've tested that with diamond and netherite pickaxes on nether quartz veins.