Running LLMs is probably the first time I find that the SoC of that generation to lack. Even Google's underpowered Tensor CPUs make a huge difference when it comes to LLM performance.

You can check your settings for GPU acceleration, it's possible that enabling that makes a big difference.

From what I've found online the difference may also simply be Snapdragon versus Exynos GPU driver optimizations, in which case I don't think the performance can be fixed by anyone but Samsung. Others online seem to get decent performance out of the model on the S21 Ultra at the very least.