When a certain investment levels (eg. 10, 50, 100, 200, 300, 400 and 500) are reached by all investments, the progress bar for all investments speed up. In "normal" mode, each of these levels increase the progress bar speed by 2.5x. This effect is similar to a profit increase achievement however this also helps the player with respect to the delay period between investments.
This concept should be kept in mind for the new player. While increases to each level provides increased revenue, the greater good may be achieved by reaching the next "All investments" achievement.
While in early game, simply reaching the Omani and Saudi fields is a huge achievement, there is an added strategy for mid-level players. Immediately after retirement, where the player is looking to take advantage of the higher revenue. However, the player may see lower revenue by clicking the Omani and Saudi fields since they move so slowly without sufficient investment levels.
In order to optimize your cash flow, get the levels of every property 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.