You need to reduce your time complexity. Right now, you are (in the worst case) looking though the entire first string for every character of the second one. This means your time complexity is O(m*n), i.e. the time your code takes rises quadratically compared to the length of your input. (m and n being the lengths of the input strings)

You should look for a different approach to the problem. The optimal solution should run in linear time.

https://en.wikipedia.org/wiki/Time_complexity