When mining a block it takes several times longer for the internal server to acknowledge it than the client itself. This is most apparent when mining with your fist. The reason for this is the internal server has huge issues with resource consumption, and runs slowly on high-end gaming PCs. This does not happen with Minecraft 1.7.10 and prior.

Increasing the JVM memory allocation up from 1024MB to 2048MB has no effect.

Steps to reproduce:

1: Create a new Minecraft world using the affected versions.
2: Break a block.

This is most apparent in amplified worlds, however, it happens with any world configuration I have tried, except flat terrain.

I have attached the relevant portion of the game output from the launcher.

This has happened consistently since the multithreading work.