Why is Minecraft running worse on an RTX 5070 Ti than my old 3060?

First check which GPU Minecraft is actually using. Open the debug screen in game and look at the renderer information, or check the GPU engine in Task Manager while the game is running.

Also make sure the monitor cable is plugged into the graphics card, not a video port on the motherboard. If the game is using integrated graphics, Windows Graphics settings can be used to select the high-performance GPU for the Java executable running Minecraft.

Install a current NVIDIA driver that supports the 5070 Ti, then restart and test again. Since this is a different computer, there is no need to remove the 3060 driver from the old machine.

Check the Windows power mode too, especially if it is set to an energy-saving option. That can affect performance or which graphics processor an application uses.

Try to narrow down whether the shader is the trigger. Check FPS without shaders, then enable them again and watch GPU usage in Task Manager. Give the world a little time to load before comparing results.

Even with the same modpack, check the actual render distance, simulation distance, resolution and shader settings on both machines. A high GPU percentage tells you the GPU is busy, but it does not by itself explain why the frame rate is so low.

Check the memory allocation for the Minecraft installation in your launcher, but don't automatically assign half your system RAM. More isn't always better. The right amount depends on the modpack and how much memory your PC has, so check the pack's recommendations if available.

That allocation is system RAM for Java, not the graphics card's VRAM. Changing it won't directly set how much VRAM the shader can use.