Block production throughput gradually degrades

[DrZoltanFazekas]

With increasing number of empty blocks produced, the block time increases gradually i.e. the block production throughput decreases. While after starting the devnet it was producing 0.94 blocks / sec, the average number of blocks / sec decreased to 0.82 after less than 3 days:

[JamesHinshelwood]

I would be keen to re-test this with #1915. This should at least tell us if its the DB or our application that is slowing things down.