This is going to be a little tough to explain but here it goes. I have a storage system with redstone, hoppers, and water. So when I turn on shaders (any shaderpack) its drops my fps from 80-100 ish down too 29-40 fps while looking at my contraption. While out in the wild away from it I get 100+ fine. I narrowed all the settings down turning them off one by one seeing what was causing it. So when I turn off shadows only (All other settings in 5 different shaders had no effect on FPS) I get around 60+ fps in the redstone storage area. I might just be dumb.

Relevant log output
No response

Minecraft Version
1.19

Iris Version
iris-mc1.19-1.2.5

Sodium Version
sodium-fabric-mc1.19-0.4.2+build.16

Operating System
Windows 10

What is your GPU?
NVIDIA Geforce RTX 1080

Java Version
Java 17

(shadows on) inside storage

(shadows on) outside

(shadows off)

This is due to how the shadow pass works.

The shadow pass has to render chunks a second time to determine what is and isn't casting a shdow, and as such has a higher performance impact than other settings.

The number of chests on screen also affects FPS quite heavily in this situation.

This should be consistent with Optifine as well.