Generally Optimized is focussed on improving performance and adding quality of life mods while keeping the vanilla experience. The pack adds most if not all features present in optifine. Many of the bellow listed features can be toggled ussing the Minecraft settings or by search the mod in the modmenu and clickig the settings icon.

Performance:

A lot of improvements you won't notice in gameplay, just higher FPS.
Lower FPS when Minecraft is in the background, this way you can keep doing stuff while the game is open.
Entity's you don't see won't render.
Small Xp clumps will become one large clump, this way there are less entity's. Mostly big Xp farms will benefit from this.
Use less cpu when starting the game.

Quality of life features:

No more ghost blocks.
A search bar in the keybindings menu.
Lower volume when Minecraft is in the background.
Your game will not close during a crash, you just rejoin and keep playing.
The option to remove chat encryption on servers that allow this to happen.
Optifine rescource packs are mostly supported.
Many different capes from different mods can be seen.
Shaders.
Zoom.
See your saturation.
An improved F3 screen.
Carpet mod.
chunk pregeneration
Invite players to your singleplayer world.
Voice chat.
Worldedit (with visual aid).
Control your gamespeed.

My other optimization packs:

Just Optimized: A simple modpack focussed on improving performance. No fancy QoL features.
Survival Optimized: A simple modpack focussed on improving performance and adding some QoL features.