After running for about an hour or two, with around 10 players on, mobs will start to freeze, be unable to inflict damage to players or other mobs. You can hit them/damage them once, but then no further. Can't build around them as the game considers that they are still there, can't push them out of the way, they don't move, etc. If you leave the area and unload the chunk, they start acting normal again. If a spawn chunk is frozen, we have to restart the server.

If you mine something in that chunk while entities are frozen, the blocks will mine and be destroyed as expected, but continue to have the animation that they were just broken and are falling to the ground.

Arrows will pause in frozen chunks, get stuck in flight.

Attempting to enderpearl in or out of a frozen chunk will result in seeing the purple particles in the target and original locations, but you don't move locations.

If I am in creative mode, and attempt to begin flying by hitting space twice while in a frozen chunk, it will not start flying. I must exit the frozen chunk.

I've tried removing plugins, adding others. No matter what, this issue persists. World was first generated starting in 1.9pre1. Chunks generated in any of the pre1-3 versions all suffer from the same issues.

Attached are screenshots from a large dig two of our players are doing. Shows how frozen the mobs are, that they can excavate around them and they stay floating. This shows a high number of entities, but we have the same issues in other areas that don't have such high entity counts.