The Square Law of Computation
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.
For example, our solar system contains millions of objections, all having gravitational effects on one another. When analysing planetary motion, astronomers start by ignoring most of these objects. (Ethan M. Rasiel, The McKinsey Way, 1999)