For settings we are using -N7 -A8m -qg.
I don't know if they are really the optimal values but I haven't found a significant improvement on these yet. I tried -qb but that was slow. I tried larger values of A but that didn't seem to make a big difference. Also -N6 didn't make much difference. Specifying H values didn't seem to make much difference. I have to admit I don't fully understand the implications of the values and was just experimenting to see what worked best. Please let me know if there's anything you suggest I should try. That said, we have already got it to a speed where it's working so I'm ok with what we now.
What settings are you using, out of interest?