Like almost any performance question of this nature, the answer is "It Depends". Specifically it depends on what exactly your code is doing and what aspect of your machine is limiting the performance. Are you doing anything else that involves reading from or writing to files on the disk (there are other functions that just load or save which would do this)?
If you are just using data in RAM and are not concerned with the initial time for MATLAB to startup, then save your money because a SSD isn't going to improve the performance of your MATLAB code greatly if you are limited by CPU, RAM, or GPU speeds.
MATLAB has an excellent built-in profiler which you can use to find out where your code is spending it's time. To use it do the following in MATLAB: