Euler 12: counting divisors of a triangle number
This is not a review, but an extended comment.

Project Euler problems are about math, not programming. To optimize, you need to do the math homework first:


The \$n\$'th triangular number is \$\dfrac{n (n+1)}{2}\$. 
A number of divisors, aka \$\sigma_0\$, is a multiplicative function. The link to divisor function may also be interesting.
\$n\$ and \$n+1\$ are coprime.


That should be enough to get you started with optimization.