Hey ! Yeah there was some bad lag problems in the demo. Even when I played it, I experienced it every time with my potato laptop. I did eventually find the reason behind it and it has since been fixed in the full version. Some people still experience lag, but it's source is different, because most people who had the issue you mentioned told me it's fine now.
As for how to reduce the lag in the demo? Well...... I don't think it's easily done as the source is coded pretty deep in the game maps.