I tend to operate with the assumption that the built-in solution will be the fastest for the general problem. If you have a non-general problem there may be faster solutions.
This implies that you will have to tell use more details, otherwise the answer will be to buy a faster computer.
Can you give an example of the code that makes str2double take 6 seconds? Or are you just calling it so often the total time spent in str2double is 6 seconds? Might it be possible to reduce the number of calls in some way?
This FEX submission was linked in the sidebar on this page. You would still have to fix the memory leak, but it claims to be faster.
