If you are talking about unofficial mods, go to where you're getting the mods and ask the questions there. This forum does not officially support unofficial mods.

If you are talking about the actual file (not in game) that are acting weird on mods, I suggest do a virus/malware scan. Many mods just add/unlock/modify the IN game stuff. It should not affect outside of your system.

If it does, that is not a mod. It is more like a virus/trojan/malware (at least that is my opinion)

Hard learned lesson (game crashes and data lost in coming)..

if the dev(s) don't issue a DLC (or authorize an Official mod) don't use it ..

otherwise they aren't what you think they are (or do you?) ..
(lagging game servers is just the half of it)

external sites that have mods are riddled with malware for cheats ..
you download as you play actively installing not only to your game folder(s)
but the server you are playing on ..
(where it aught NOT causes game lag for that server and any that connect to it)

they wont just help with cheating in game ..

they will help themselves to cheat you out of your account data ..

as well as those you play with ..

will be affected as well ..

and steam doesn't help by allowing it <<<
~known fact(s)