So many people contact me asking about adding something like this to my addon and it's been spammed all over the forums a couple of times. Block breaking is client side. The client sends a packet when it starts mining, changes tools/aborts during mining and when it finishes mining. Having a constant system spamming packets to the server saying that the player is mining a block is ridiculous, which is why Mojang doesn't have that as a feature.
You can't change how fast a player mines a block. Also if you're giving this to donators, that is against Mojang's EULA which gives Mojang the rights to shut your server down if they want. If all players have the ability to access this feature, you won't have to worry. "Donators can not have gameplay-affecting features over other players" - Mojang
Mojang's EULA if interested
Here and
Here
(EULA has been around since 2014 and was put into action summer 2015)
The only way to "Simulate" block breaking is constantly cancel the
on block damage or
on break event and for every X hits of the block. Add 1 to a variable of the block. Then if that variable goes over X break the block. You can add the block crack effect or the block break packet to the block per stage to add an effect of breaking to it aswell.