On 1.8 optifine on servers whilst streaming, I get 400 fps.

On sodium 1.16, speedrunning I get 40 fps. Can anyone help?

Comparing 1.8 performance to 1.16 performance is unfair, especially since you're comparing playing on servers to playing locally. 1.16 is more hardware intensive than 1.8, and playing on a server is less hardware intensive than locally speedrunning. You should be comparing speedrunning on 1.16 vanilla against speedrunning on 1.16 with sodium et al.