I’ve recently started to participate in this site Project Euler. It’s a website with a series of mathematics/programming problems, ranging from easy to very hard. Some problems, if not solved using the right algorithm, can take more than 5 minutes to get the result. So it only matters if you use correct and efficient algorithms. These problems are meant to be solved in only less than a minute.
Check out my Git repository of my solutions (mostly in Python because I find it faster and neater to code) to some of the Euler questions: baopham/projectEuler