When destroying a block i have to wait 'til it disappears to get the item.
What I expected to happen was...:
Destroying a block and getting the item afterwards.
What actually happened was...:
Destroying a block and having to wait double the time 'til it disappears to get the item.
It's called "block lag", if you are playing on a server, it's just server cannot keep up with all the players and what they are doing, if you are playing in singleplayer, your PC is just not strong enough. Try lowering your render distance to 4-5 chunks, lower other setting. It should work