UPDATE: Per one of Conan's suggestions on Steam chat (turns out we were friends this whole time), I renamed my addons folder to something else, and ran a pseudo-clean-install of Blockland. The mouse lag in the menu alone was the worst I've seen so far. So, it appears I can't even run a clean install properly anymore.
Again, I haven't made any significant changes to either my computer's settings or my Blockland files in recent memory; the only change I've made to the latter directly was removing someone from my trust list using the config file. That's it, as far as I can remember.
Also, the lag isn't constant, as I may have implied in my original post, but it is pretty bad when it does crop up.