Can run RLCraft and its shader on 60fps but not shaders in optifine (base game)

[Craft3324]

All the times i've played RLCraft i have had no issues with frame rate or lag despite it using a shader. But whenever i try to run shaders through the normal game together with Optifine i get below 30fps and at worst around 15fps. 

Why am i getting such a significant difference in fps and is it possible for me to also get a good frame rate when running shaders in the base game?

[Hipposgrumm]

Modern versions of Minecraft are running on an entirely new engine that may take up more resources. Additionally, this is not a specifically Forge-related question, so I will give no more detail.