To answer your specific question. I have got a couple of Cambodia visas by mailing my passport to their Washington DC embassy. In each case they turned it around within one business day. They seem efficient at handling visa apps.

And in my case I did it through DC because them %$$#%&#@ at the Koh Kong crossing will still try to shake you down for a few extra Baht and I get satisfaction out of depriving them the money. Arriving by air though, there is really no reason not to just get it on arrival since they are honest at the airports.