Remove Duplication Vulnerabilities

Remove CraftBukkit "Feature" of ItemStack's having infinite use if less than 0 stack size.
Re-enable low priority queue, use on all chunk sends
Reduce compression to save CPU time
Fix map in itemframes lag
Enable Improved ping sending
Metrics is not severe
Fix livingentity collision reduction
Timings improvements
Entity Activation Range

This feature gives 3 new configurable ranges that if an entity of the matching type is outside of this radius of any player, will tick at 5% of its normal rate.

This will drastically cut down on tick timings for entities that are not in range of a user to actually be "used".
This change can have dramatic impact on gameplay if configured too low. Balance according to your servers desired gameplay.
Reduce Entity Tracking Ranges

This will let you configure how far to track entities in range from players, so that the entity does not render on the client if out of this range.
This has multiple benefits:

1) Less bandwidth. Not sending update packets for entities that are not even close to a player, or even close enough to clearly see.
2) Less lag by maps in item frames - Default range is 160 blocks... Many players can track that item frame and cause lag and not even see it.
3) Less lag in general - Less work for the server to do
4) Less client lag - Not trying to render distant item frames and paintings and entities will reduce entity count on the client, which is major for shop/town worlds which may use tons of item frames.
Improved tile entity lookup for chunk sending

Instead of scanning the entire worlds tile entities to find out what tile entities are in the chunk... just use the arraylist of tile entities we already have.
This results in a good reduction of time spent in player ticking.
Optimize getTileEntities

Avoid traversing tile entities for every loaded chunk