For every component of a system - for every additional equation in a problem - the amount of computation required to solve the system increases at least as fast as the square of the number of equations. 

In other words, the complexity to solve a problem grows at least as fast as the square of the number of things you are trying to solve. (Gerald Weinberg, An Introduction to General Systems Thinking, 1975). 

So if I try to solve 4 things at once - as opposed to solving each of those 4 things individually, the relative complexity is 4x4=16 versus 4. 

In practice: If the complexity of your problem doubles, the time it takes to solve it quadruples - unless you make some simplifications. Many factors affect your business. Focus on the most important ones - the key drivers.