There are many code level optimizations that can make an application perform better in specific areas. Consider using different algorithms and data structures. Changing these can make large performance gains.
Consider what problems you are trying to solve and see if you can solve them in different ways. For example:
if you are modelling a landscape as a grid, an alternative is an irregular mesh.
if you are representing object positions by placing them in a grid, an alternative is to use a mathematical approach.
if you are computing collision detection in three dimensions, consider if the third dimension can be approximated or ignored. This can reduce many types of computations.
These are examples. Experiment with different approaches and measure the results to see how well they work.
If you cannot think of alternative approaches, describe the problem to someone else and see how they would solve it. If their solution is different then that is an alternative you can consider.
The act of describing the solution can be sufficient to bring new ideas to mind. This can assist you in thinking about possible alternatives.