There are a few potential approaches you can take to improve the performance of your code:

1. Optimize the method that adds spaces: Instead of adding spaces one at a time, you could try adding them all at once. This would require calculating the number of spaces needed and adding them in a single operation.

2. Use a different data structure for storing the output string: Appending to a string can be inefficient, especially if you're doing it repeatedly. You could consider using a different data structure, such as a list of strings, and then join them together at the end.

3. Use a different algorithm for dividing the input string into substrings: Depending on the specifics of your input, you may be able to come up with a more efficient way to divide the input string into substrings of the desired width.

4. Consider using multiple threads or processes to parallelize the work: If the input string is very large, you may be able to speed up the process by using multiple threads or processes to work on different parts of the input string simultaneously.