So, my server is having ridiculous amounts of server side lag. Trying to eat a single food? It'll take over a minute (no exaggeration, seriously). Trying to kill a single mob? It'll start teleporting around and your swings will just hit air. Trying to craft a single item? It'll get stuck in the crafting grid for all eternity.
I've read the article on lag, I've searched forum after forum, but never found working solutions. The console keeps being spammed with "[Server thread/WARN] [minecraft/MinecraftServer]: Can't keep up! Did the system time change, or is the server overloaded? Running *~1000-4000*ms behind, skipping *~500-1000* tick(s)" which obviously means it recognizes the problem, but it's not giving any more information.
I'm running 1.12.2 Forge, one world, and only a couple of mods installed.
Edit: it seems I need to say this again, this is server-side lag. The clients are fine. Stuff like OptiFine isn't gonna help.