(Having issues with mobs despawning right next to you? Turn up your World Simulation distance, it is NOT the addon doing it)
Stricter Entity Despawn
Mobs too far away or off screen will now despawn quicker! (It will despawn mobs in loaded chunks after 3 mins and 20 seconds, to ensure mobs aren't disappearing right next to you. Only mobs too far away or in unloaded chunks will despawn right away!)
Entities like arrows will despawn after 10 seconds.
Mobs that WONT despawn are Bosses like the Ender Dragon, nametagged mobs or ANY tamed mobs.
(Mobs with more then 130 hearts won't be despawned by the addon because it will assume its a type of boss. Furthermore animals won't despawn if close together so mob farms still work)
If you need specific Entities to not despawn, such as addon mobs, then there is a config file you can find by opening up the pack folders! You change stuff such as adding mobs to the Do Not Despawn Blacklist.
This system already supports addons like Better On Bedrock, Darkage Bizarre, Billey Mobs, Mythological Craft, The Forgotten Legends, Nico Dungeons, RPG addon by Kubo and Alyica Dungeons so no important mobs are lost.

Item Merging
Dropped items will now group up, offering a slight increase in performance. (XP Orbs won't be clumped as there is no performance benefits to that)
Optimized Particles
You won't notice much of a visual difference as particles are reduced. For example, when you land a crit hit, instead of scattering 20 particles, it only scatters 10. It is not noticeable potato, but minor fps gain.
Optimized Game Logic
Falling gravel will despawn after a few seconds if still falling.