The Glad Giraffe update (Dec 8th) that added lots of new fun things to Starbound must have added some unintended bugs, in this video I talk about a temporary solution.
Some notes:
~Although Starbound is lightweight, if you have always had lag while hosting and playing it might just be your PC.
~Telling your friends to turn off v-sync too could also help them as well even if they aren't hosting.
~Backup and verifying your game files (steam) could fix the problem as well if it has persisted for awhile.
~A lot of the mods/people on the forums will say "Don't play modded" if you say you're experiencing lag, that isn't much of an answer so hopefully this video helps. (I've personally never had lag with some of the major mods).
~This will fix singleplayer lag (only when it occurs because of this) as well.