The more times businesses are upgraded, the more money you will earn. Consider evening everything out and do not focus on a single business. Once reaching level 25 to 50 you can begin the multitude of upgrades for achievements and improvements. The first screen of the "Unlocks" shows the number of upgrades you must purchase for each business in order to greatly improve its speed or profits. After a requirement has been met, a new one will be unlocked. Upgrade before the timer runs out. Later businesses that start appearing with the Hockey Stadium require some time before you can collect their profits. However if you upgrade them a few times just before the timer is ready, you still get the larger amount of cash. Spend your money for upgrading other areas then invest in those large businesses when the timer gets close to resetting.