Mined blocks regenerate/reappear; major lag (5s+)

Single player - survival mode - new map created in ver 1.8.4

On entering the Jungle, lag was apparent immediately with two slo-mo zombies. Later when trying to mine, none of the blocks would break unless continuing with the 'bashing' for an additional 5s and then another 5s later the block would be free to collect (with an additional lag).

This is making the game unplayable...