Problem here is the collisions algorithm, it may require large computing times specially using higher densities. Check your density settings (Distribution Map->X/Y Size), usually you will get the same result lowering it, when most of items are discarded (check General->Forest stats for details).
Anyway, usually it's not necessary to use collisions, specially to create grass. A simple setting of "Full" distribution map, and Random Translation to avoid the grid effect, should be enough. If you need to exclude other items for the grass, use the Areas