how big is ur scene. in Previous Vue whn u load a terrain, the size of terrain by default is 256 * 256. while in VUE 10 size of terrain by default is 1Km * 1km, thus it uses hell lot of memory n time to populate. try reducing the size of terrain and then populate with very low density.  if it responds well, go on slowly incresing the pop. which u wan.

do try ths.