At certain levels (10, 50, 100, 200, 300, 400 and 500), the achievement for "All Properties" will speed up the rate at which properties earn their money.
Once a business plan has been put into place, the time for an investment to earn one more "tick" is fixed.
Optimization for this Effect
This tip is primarily intended for immediately after resets when you have higher amounts of experience.
In order to optimize your cash flow, do not start the Omani and Saudi fields right away, get the levels of every investment as high as possible before starting. This will apply a large speed bonus from achievements, and the first tick will run much faster than if you start them at level 1.