Why do I have so little FPS?

I have a relatively good PC that I built myself a year ago. At the beginning it had around 1000 fps in Minecraft and I even had a shader on.

I started Minecraft for the first time a few days ago, and I've got around 100-150 fps. And sometimes only around 30. But why is that?

Can someone help me?

Do you use the 1.16.4? Unfortunately, it is very inperforming.

No, but 1.9