You left out some major information. If you are from Somalia or North Korea you are going to be waitng a looong time.
However, the Cambodian Visa can be done on arrival at the border if you are coming from Thailand. Many provate tour companies will do this process for you. I will warn you now, many tour companies will charge you extra money for the visa ...
I am a US citizen abd visited Siem Reap in December 2018. I got my visa on arrival. It took less than half hour. The process seemed very systematic and efficient. I carried a photograph for the application; I recommend that. Also carry US$30.
Hope this helps.
Almost four years later, the rule hasn't changed. My direct experience of these days and some recent references I read confirm the schema:
1000 KHR = 25 cents
2000 KHR = 50 cents
4000 KHR = 1 $
5000 KHR = 1,25 $
10000 KHR = 2,5 $
20000 KHR = 5 $
No app and no more is needed in this basic table of conversion!