But Starbound doesn't use GPU, so having vsync means 'your CPU needs to do more work'. Why it helps: 99% of games don't suffer from vsync being enabled (in fact, it's better for them), because vsync is normally done by GPU.
Edit it, replacing the line 'vsync' : true, (which is slow) with 'vsync' : false, (which is faster).
These methods are known to improve game speed: However, things like ITDs can also contribute to the lag. So regardless of how good your computer is, you might still run into problems. It also doesn't use the GPU (all graphics are calculated by CPU).
It only uses 1 CPU core for server (in singleplayer too), and only 1 CPU core for client. Starbound (by itself, even without any mods) is very poorly optimized. 7 If Starbound takes 5+ minutes to start.