Project Euler #14 - Longest Collatz sequence
While many of the Project Euler problems can be solved by brute force, this often isn't the fastest way to do them. Usually there is some kind of algorithmic insight that you can apply that will make your solution faster. This kind of insight is independent of programming language - simply running your code through a hypothetical "go-fast compiler" still won't match the speed of the correct algorithm.

In your case, here's a hint. Many of the sequences you calculate look the same as ones you've previously done. How could you use that information to speed up your algorithm?