OptiFine

Optifine is a Mod founded many years ago, with the goal of making Minecraft more performant. And it succeeded for a while.

Back in the 1.12 and earlier days, Optifine was a great way to make Minecraft run better, and it had support for Shaders. But nowadays, it is entirely irrelevant, and actually makes the game run worse.
On modern versions, the performance benefits are miniscule to unnoticeable, and loading the game or worlds take at minimum 3x as long.

You're much better off using mods like Embeddium and ModernFix (they can be installed together!), or for older versions, VintageFix (1.12.2) or Archaic Fix (1.7.10).
If you want to use Shaders, check out the guide here: What Are Shaders?