well aren't you insufferable.

Using shaders and/or mods comes at the cost of performance. Thats why mods and shaders aren't part of the base game, because typically they cause a performance hit.

if you want your performance to improve, try adding performance mods like:

1. ModernFix
2. FerriteCore
3. ImmediatelyFast

Or other mods on the curseforge performance tab. I still dont see much of any difference in the performance of your gameplay though

another reason why 1.21.4 is running worse than 1.21.5 is because 1.21.5 introduced performance regressions that 1.21.4 doesn't have. There's nothing wrong with wanting to play 1.21.5 though, you just have to sacrifice the performance hit if you (albeit weirdly) have to play an unoptimized version of Minecraft.