Pixelmon Mod

All of these optimization mods work with OptiFine:

VanillaFix (Increases FPS): https://www.curseforge.com/minecraft/mc-mods/vanillafix

Phosphor (Optimizes lighting engine, speeds up dimensional travels): https://www.curseforge.com/minecraft/mc ... phor-forge

Clumps (Reduces lag caused by xp orbs): https://www.curseforge.com/minecraft/mc-mods/clumps

FastWorkbench and FastFurnace (Optimizes crafting and smelting):
https://www.curseforge.com/minecraft/mc ... tworkbench
https://www.curseforge.com/minecraft/mc ... astfurnace
NOTE: Change "Disable Recipe Book"=true to false in FastBench.cfg to keep Recipe Book enabled.

Xp orbs are barely ever a thing with Pixelmon, so Clumps wouldn't do much.