I performed the following experiment, in each case counting the credits earned and number of levels that were open and selling items. Note that my elevator is the 10PPM model:
- Play TDS for 10 minutes, using the elevator and keeping only the top item (3 credit item) in stock for each level.
- Turn TDS off and wait ten minutes.
Results:
- 4,420 credits using 31 active levels (PPM: 4.75)
- 8,187 credits using 27 active levels (PPM: 10.11)
These two tests were performed consecutively. It appears that TDS either uses a different model to earn credits while you are actively playing the game vs when it restarts and calculates all the expected earnings.
The difference is staggering, and if you're goal is credits you should be playing for brief periods of time infrequently, rather than constantly. You won't be able to make many bux that way, though.
This is from tests conducted after the 1.3 update in early February 2014, and this may change over time, particulalry since it appears to be a bug - you should be earning according to your elevator speed in the game as well as out of game. Tested on iOS - I expect it will be the same on other platforms, but it's possible that the timing code is platform specific, so it might be different. Would be worth more testing.