If you are crashing on technic please dedicate at least 4gb of RAM, if you still crash after that, dedicate some more RAM. If you're still crashing, remove the following mods: Custom Main Menu, Better Loading Screen, ambience mod, ITLT mod, and Resource Loader. Also delete the following folders: Resources.
If you're lagging, delete the Better Foliage mod, it seems to be the cause of some player's lag, not Galacticraft.