When a server sends a Change Game State packet (0x1E as of 1.16.1) with reason 7 (Rain level change) and the value is 1 or below, the game runs at maximum FPS, but as we send higher values, the FPS get lower. With value 40, I get 20 FPS. With value 45, I get 10 FPS. And with value 50, I get 2-3 FPS. And, finally, with a value as high as 1000, the game freezes completely and is effectively crashed as it will not recover.
As of 1.16.1, values above 20 also cause font color changes which persist (even to the main menu) until a normal value is sent.