When sand or gravel is unsupported by a block beneath it, it falls to the ground. This is achieved using a block update. When sand or gravel receives a block update, it checks to see if it has a block underneath it. If it does not, it disappears and creates a sand or gravel entity. This then moves to the ground and disappears, producing a solid sand or gravel block on supporting ground. This process usually takes about one tick (0.1 seconds) to calculate.

However, this process is heavily delayed. In some cases it can take up to 3 seconds for an unsupported sand or gravel block to respond to this. Further, it can take the same amount of time to revert to a sand or gravel block again.

This can be caused by numerous problems, however the most obvious and also the most common is lag, which can be located on the client, server or both.

On the client side it can be caused by congestion, where numerous programs are running at once and causing a performance spike. It can also be caused by router congestion where internet download and upload speeds are reduced because of the high use if internet-related tasks.

On the server side this lag can be caused by congestion also, where high amounts of players are connected. Server side lag can also be caused often by excessive redstone use, which can be seen most easily in servers that control functions using clocks and complex circuits.

In conclusion, entity delay is mostly a lag-related effect but many other causes are present.