. Really collisions have a great impact on performance because two issues:
1) Camera's clipping is much less efficient. Without collisions Forest can exclude large areas from scattering (at least, everything behind the camera), but using collisions the full area must be processed, excluding the items on a second phase.
2) Beyond certain value, although you increase the density of the Distribution map, the number of visible items doesn't change significantly (because most of them are removed because collisions). Instead, the scattering process time grows exponentially.
So, for dense distributions (as grass) i suggest to use a Full Distribution map, collisions disabled and Random Translation enabled (to avoid the grid look). Then, you can adjust the density to get the desired result. The grass tutorial includes a sample scene using these parameters.