[2020] FPS sudden decrease when doing network?

You can start by checking this list of broken and incompatible mods:
https://docs.google.com/spreadsheets/d/1mVFkj_7ij4FLzKs2QJaONNmb9Z-SRqUeG6xFGqEX1ew/edit?usp=sharing

If you don't have any there, you are likely gonna have to go through your mods to find the culprit. Start with mods that have something to do with networks, and make sure to restart the game between each test.