Just a note on the side:

I experience this Bug on android,on switch,on pc.

I also know this Bug since minimum a year. so it is in many versions the same "problem".

When i have a efficiency IV or V pickaxe i still mine in the speed of like efficiency III.

But when i move away from the blocks and start mining outside the block reach distance and walk towards the blocks i want to mine, then it mines at (full speed i suppose?)

else it is VERYSLOW.