I built a gaming pc and e.g. Minecraft ran without shaders at 400 fps and it felt really smooth and now (a few hours later) I have around 100 fps and it no longer feels smooth
It is best to reinstall MC again
Do you have a processor with an integrated graphics unit + a dedicated graphics card?
If so, it may be that the MC does not run on the significantly more powerful graphics card.
So here are my tips that I give most people:
Reinstall the game
Download the graphics card driver from the AMD / Nvidia site again
Download chipset drivers from the mainboard manufacturer's website.
Clean the PC fan
See what's going on in the background. The task manager opens. How much RAM is free?
Limiting the FPS number when playing makes sense everything above 120fps is only useful for paid professionals and makes the PC run hot and costs electricity
I already have but somehow changes nothing
Ok then make more RAM available to Minecraft. Google simply how to allocate more RAM to Minecraft
Could it be because I don't have to activate Windows?
No do what I told you