Hi, we made several tests, but could not reproduce that difference between Edge and Point mode.
These are our results with one of the tests scenes (you can find it attached):
Edge Mode, Corona 1.7.2, 15 passes: 00:52
Point Mode, Corona 1.7.2, 15 passes: 00:45
Edge Mode, Corona 1.6.3, 15 passes: 00:55
Point Mode, Corona 1.6.3, 15 passes: 00:48
Edge mode is a bit slower but not much more. That's normal because Corona must render the hidden grass blades as transparent items.
Perhaps render time may higher using other materials, i don't know.
As said before, there is not much we can do, because rendering process is out of our control.
If the Corona team replies something new in their forum, please tell us.
Thanks,