PolyReflex

By default, Minecraft allows up to two frames to render on the CPU when the GPU is busy. These effectively are "silent" frames, where mouse movements, inputs, and more are registered, but don't show up on your screen.

PolyReflex interacts with your GPU to implement NVIDIA Reflex or AMD Anti-Lag 2 directly in Minecraft, which times up frames with the GPU to get you the lowest input latency. Unlike other mods which claim to implement Reflex / Anti-Lag 2, this actually uses data from the GPU, meaning the effect will be significantly better.

This will mean your FPS may go down. This is intended behavior.